cli: unblock "jj git init --colocate" in existing Git repo directory

I'm not sure what's the conclusion in jj-vcs#2747, but I don't think there is a
disagreement on allowing --colocate to import existing Git repo.

CHANGELOG.md

@@ -66,6 +66,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * When creating a new workspace, the sparse patterns are now copied over from
   the current workspace.
 
+* `jj git init --colocate` can now import an existing Git repository. This is
+  equivalent to `jj git init --git-repo=.`.
+
 * `jj git fetch` now automatically prints new remote branches and tags by default.
 
 * `--verbose/-v` is now `--debug` (no short option since it's not intended to be used often)

cli/src/cli_util.rs

@@ -1818,7 +1818,7 @@ fn map_workspace_load_error(err: WorkspaceLoadError, workspace_path: Option<&str
                     message,
                     "It looks like this is a git repo. You can create a jj repo backed by it by \
                      running this:
-jj git init --git-repo=.",
+jj git init --colocate",
                 )
             } else {
                 user_error(message)

cli/src/commands/git.rs

@@ -125,12 +125,9 @@ pub struct GitInitArgs {
     /// `git` repo, allowing the use of both `jj` and `git` commands
     /// in the same directory.
     ///
-    /// This is done by placing the backing git repo into a `.git` directory
-    /// in the root of the `jj` repo along with the `.jj` directory.
-    ///
-    /// This option is only valid when creating new repos. To
-    /// reuse an existing `.git` directory in an existing git
-    /// repo, see the `--git-repo` param below.
+    /// This is done by placing the backing git repo into a `.git` directory in
+    /// the root of the `jj` repo along with the `.jj` directory. If the `.git`
+    /// directory already exists, all the existing commits will be imported.
     ///
     /// This option is mutually exclusive with `--git-repo`.
     #[arg(long, conflicts_with = "git_repo")]
@@ -379,52 +376,67 @@ pub fn git_init(
     colocate: bool,
     git_repo: Option<&str>,
 ) -> Result<(), CommandError> {
-    if colocate {
-        let (workspace, repo) = Workspace::init_colocated_git(command.settings(), workspace_root)?;
-        let workspace_command = command.for_loaded_repo(ui, workspace, repo)?;
-        maybe_add_gitignore(&workspace_command)?;
-        return Ok(());
+    #[derive(Clone, Debug)]
+    enum GitInitMode {
+        Colocate,
+        External(PathBuf),
+        Internal,
     }
 
-    if let Some(git_store_str) = git_repo {
-        let git_store_path = command.cwd().join(git_store_str);
-        let (workspace, repo) =
-            Workspace::init_external_git(command.settings(), workspace_root, &git_store_path)?;
-        // Import refs first so all the reachable commits are indexed in
-        // chronological order.
-        let colocated = is_colocated_git_workspace(&workspace, &repo);
-        let repo = init_git_refs(ui, command, repo, colocated)?;
-        let mut workspace_command = command.for_loaded_repo(ui, workspace, repo)?;
-        maybe_add_gitignore(&workspace_command)?;
-        workspace_command.maybe_snapshot(ui)?;
-        if !workspace_command.working_copy_shared_with_git() {
-            let mut tx = workspace_command.start_transaction();
-            jj_lib::git::import_head(tx.mut_repo())?;
-            if let Some(git_head_id) = tx.mut_repo().view().git_head().as_normal().cloned() {
-                let git_head_commit = tx.mut_repo().store().get_commit(&git_head_id)?;
-                tx.check_out(&git_head_commit)?;
-            }
-            if tx.mut_repo().has_changes() {
-                tx.finish(ui, "import git head")?;
-            }
+    let colocated_git_repo_path = workspace_root.join(".git");
+    let init_mode = if colocate {
+        if colocated_git_repo_path.exists() {
+            GitInitMode::External(colocated_git_repo_path)
+        } else {
+            GitInitMode::Colocate
         }
-        print_trackable_remote_branches(ui, workspace_command.repo().view())?;
+    } else if let Some(path_str) = git_repo {
+        GitInitMode::External(command.cwd().join(path_str))
     } else {
-        if workspace_root.join(".git").exists() {
-            let cwd = command.cwd().canonicalize().unwrap();
-            let relative_wc_path = file_util::relative_path(&cwd, workspace_root);
+        if colocated_git_repo_path.exists() {
             return Err(user_error_with_hint(
                 "Did not create a jj repo because there is an existing Git repo in this directory.",
-                format!(
-                    r#"To create a repo backed by the existing Git repo, run `jj git init --git-repo={}` instead."#,
-                    relative_wc_path.display()
-                ),
+                "To create a repo backed by the existing Git repo, run `jj git init --colocate` \
+                 instead.",
             ));
         }
+        GitInitMode::Internal
+    };
 
-        Workspace::init_internal_git(command.settings(), workspace_root)?;
+    match &init_mode {
+        GitInitMode::Colocate => {
+            let (workspace, repo) =
+                Workspace::init_colocated_git(command.settings(), workspace_root)?;
+            let workspace_command = command.for_loaded_repo(ui, workspace, repo)?;
+            maybe_add_gitignore(&workspace_command)?;
+        }
+        GitInitMode::External(git_repo_path) => {
+            let (workspace, repo) =
+                Workspace::init_external_git(command.settings(), workspace_root, git_repo_path)?;
+            // Import refs first so all the reachable commits are indexed in
+            // chronological order.
+            let colocated = is_colocated_git_workspace(&workspace, &repo);
+            let repo = init_git_refs(ui, command, repo, colocated)?;
+            let mut workspace_command = command.for_loaded_repo(ui, workspace, repo)?;
+            maybe_add_gitignore(&workspace_command)?;
+            workspace_command.maybe_snapshot(ui)?;
+            if !workspace_command.working_copy_shared_with_git() {
+                let mut tx = workspace_command.start_transaction();
+                jj_lib::git::import_head(tx.mut_repo())?;
+                if let Some(git_head_id) = tx.mut_repo().view().git_head().as_normal().cloned() {
+                    let git_head_commit = tx.mut_repo().store().get_commit(&git_head_id)?;
+                    tx.check_out(&git_head_commit)?;
+                }
+                if tx.mut_repo().has_changes() {
+                    tx.finish(ui, "import git head")?;
+                }
+            }
+            print_trackable_remote_branches(ui, workspace_command.repo().view())?;
+        }
+        GitInitMode::Internal => {
+            Workspace::init_internal_git(command.settings(), workspace_root)?;
+        }
     }
-
     Ok(())
 }
 

cli/tests/test_git_init.rs

@@ -542,12 +542,29 @@ fn test_git_init_colocated_via_flag_git_dir_exists() {
     let workspace_root = test_env.env_root().join("repo");
     init_git_repo(&workspace_root, false);
 
-    let stderr = test_env.jj_cmd_failure(&workspace_root, &["git", "init", "--colocate"]);
+    let (stdout, stderr) =
+        test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "--colocate", "repo"]);
+    insta::assert_snapshot!(stdout, @"");
     insta::assert_snapshot!(stderr, @r###"
-    Error: Failed to access the repository
-    Caused by:
-    1: Failed to initialize git repository
-    2: Refusing to initialize the existing '$TEST_ENV/repo/.git' directory
+    Done importing changes from the underlying Git repo.
+    Initialized repo in "repo"
+    "###);
+
+    // Check that the Git repo's HEAD got checked out
+    let stdout = test_env.jj_cmd_success(&workspace_root, &["log", "-r", "@-"]);
+    insta::assert_snapshot!(stdout, @r###"
+    ◉  mwrttmos [email protected] 1970-01-01 01:02:03.000 +01:00 my-branch HEAD@git 8d698d4a
+    │  My commit message
+    ~
+    "###);
+
+    // Check that the Git repo's HEAD moves
+    test_env.jj_cmd_ok(&workspace_root, &["new"]);
+    let stdout = test_env.jj_cmd_success(&workspace_root, &["log", "-r", "@-"]);
+    insta::assert_snapshot!(stdout, @r###"
+    ◉  sqpuoqvx [email protected] 2001-02-03 04:05:07.000 +07:00 HEAD@git f61b77cd
+    │  (no description set)
+    ~
     "###);
 }
 

cli/tests/test_global_opts.rs

@@ -210,7 +210,7 @@ fn test_no_workspace_directory() {
     insta::assert_snapshot!(stderr, @r###"
     Error: There is no jj repo in "."
     Hint: It looks like this is a git repo. You can create a jj repo backed by it by running this:
-    jj git init --git-repo=.
+    jj git init --colocate
     "###);
 }
 

cli/tests/test_init_command.rs

@@ -497,7 +497,7 @@ fn test_init_git_internal_must_be_colocated() {
     let stderr = test_env.jj_cmd_failure(&workspace_root, &["init", "--git"]);
     insta::assert_snapshot!(stderr, @r###"
     Error: Did not create a jj repo because there is an existing Git repo in this directory.
-    Hint: To create a repo backed by the existing Git repo, run `jj git init --git-repo=.` instead.
+    Hint: To create a repo backed by the existing Git repo, run `jj git init --colocate` instead.
     "###);
 }
 

docs/git-comparison.md

@@ -100,8 +100,7 @@ parent.
   <tbody>
     <tr>
       <td>Create a new repo</td>
-      <td><code>jj init --git</code> (without <code>--git</code>, you get a
-          native Jujutsu repo, which is slow and whose format will change)</td>
+      <td><code>jj git init [--colocate]</code></td>
       <td><code>git init</code></td>
     </tr>
     <tr>

docs/git-compatibility.md

@@ -96,8 +96,8 @@ into a directory by the same name.
 ## Co-located Jujutsu/Git repos
 
 A "co-located" Jujutsu repo is a hybrid Jujutsu/Git repo. These can be created
-if you initialize the Jujutsu repo in an existing Git repo by running `jj init
---git-repo=.` or with `jj git clone --colocate`. The Git repo and the Jujutsu
+if you initialize the Jujutsu repo in an existing Git repo by running `jj git
+init --colocate` or with `jj git clone --colocate`. The Git repo and the Jujutsu
 repo then share the same working copy. Jujutsu will import and export from and
 to the Git repo on every `jj` command automatically.
 

docs/github.md

@@ -66,12 +66,11 @@ changes.
 
 ## Working in a Git co-located repository
 
-After doing `jj init --git-repo=.`, Git will be in
-a [detached HEAD state][detached], which is unusual, as Git mainly works with
-branches. In a co-located repository, every `jj` command will automatically
-synchronize Jujutsu's view of the repo with Git's view. For example,
-`jj commit` updates the HEAD of the Git repository, enabling an incremental
-migration.
+After doing `jj git init --colocate`, Git will be in a [detached HEAD
+state][detached], which is unusual, as Git mainly works with branches. In a
+co-located repository, every `jj` command will automatically synchronize
+Jujutsu's view of the repo with Git's view. For example, `jj commit` updates the
+HEAD of the Git repository, enabling an incremental migration.
 
 ```shell
 $ nvim docs/tutorial.md