cli: ensure default remote branch is imported by "git clone"

I originally thought this would be unavoidable, but I was wrong. "jj git clone"
doesn't implicitly create any local branch if git.auto-local-branch is off, and
that's fine because the detached HEAD state is normal in jj.

That being said, Git users would expect that the main/master branch exists.
Since importing the default branch is harmless, let's create and track it no
matter if git.auto-local-branch is off.

cli/src/commands/git.rs

@@ -507,6 +507,10 @@ fn cmd_git_clone(
             .get_remote_branch(default_branch, remote_name);
         if let Some(commit_id) = default_branch_remote_ref.target.as_normal().cloned() {
             let mut checkout_tx = workspace_command.start_transaction();
+            // For convenience, create local branch as Git would do.
+            checkout_tx
+                .mut_repo()
+                .track_remote_branch(default_branch, remote_name);
             if let Ok(commit) = checkout_tx.repo().store().get_commit(&commit_id) {
                 checkout_tx.check_out(&commit)?;
             }

cli/tests/test_git_clone.rs

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use std::path::{self, PathBuf};
+use std::path::{self, Path, PathBuf};
 
 use crate::common::TestEnvironment;
 
@@ -213,11 +213,8 @@ fn test_git_clone_colocate() {
     "###);
 
     // The old default branch "master" shouldn't exist.
-    let stdout = test_env.jj_cmd_success(
-        &test_env.env_root().join("clone"),
-        &["branch", "list", "--all"],
-    );
-    insta::assert_snapshot!(stdout, @r###"
+    insta::assert_snapshot!(
+        get_branch_output(&test_env, &test_env.env_root().join("clone")), @r###"
     main: mzyxwzks 9f01a0e0 message
       @git: mzyxwzks 9f01a0e0 message
       @origin: mzyxwzks 9f01a0e0 message
@@ -309,3 +306,59 @@ fn test_git_clone_colocate() {
     Error: Destination path exists and is not an empty directory
     "###);
 }
+
+#[test]
+fn test_git_clone_remote_default_branch() {
+    let test_env = TestEnvironment::default();
+    let git_repo_path = test_env.env_root().join("source");
+    let git_repo = git2::Repository::init(git_repo_path).unwrap();
+    set_up_non_empty_git_repo(&git_repo);
+    // Create non-default branch in remote
+    let oid = git_repo
+        .find_reference("refs/heads/main")
+        .unwrap()
+        .target()
+        .unwrap();
+    git_repo
+        .reference("refs/heads/feature1", oid, false, "")
+        .unwrap();
+
+    // All fetched branches will be imported if auto-local-branch is on
+    test_env.add_config("git.auto-local-branch = true");
+    let (_stdout, stderr) =
+        test_env.jj_cmd_ok(test_env.env_root(), &["git", "clone", "source", "clone1"]);
+    insta::assert_snapshot!(stderr, @r###"
+    Fetching into new repo in "$TEST_ENV/clone1"
+    Working copy now at: sqpuoqvx cad212e1 (empty) (no description set)
+    Parent commit      : mzyxwzks 9f01a0e0 feature1 main | message
+    Added 1 files, modified 0 files, removed 0 files
+    "###);
+    insta::assert_snapshot!(
+        get_branch_output(&test_env, &test_env.env_root().join("clone1")), @r###"
+    feature1: mzyxwzks 9f01a0e0 message
+      @origin: mzyxwzks 9f01a0e0 message
+    main: mzyxwzks 9f01a0e0 message
+      @origin: mzyxwzks 9f01a0e0 message
+    "###);
+
+    // Only the default branch will be imported if auto-local-branch is off
+    test_env.add_config("git.auto-local-branch = false");
+    let (_stdout, stderr) =
+        test_env.jj_cmd_ok(test_env.env_root(), &["git", "clone", "source", "clone2"]);
+    insta::assert_snapshot!(stderr, @r###"
+    Fetching into new repo in "$TEST_ENV/clone2"
+    Working copy now at: pmmvwywv fa729b1e (empty) (no description set)
+    Parent commit      : mzyxwzks 9f01a0e0 feature1@origin main | message
+    Added 1 files, modified 0 files, removed 0 files
+    "###);
+    insta::assert_snapshot!(
+        get_branch_output(&test_env, &test_env.env_root().join("clone2")), @r###"
+    feature1@origin: mzyxwzks 9f01a0e0 message
+    main: mzyxwzks 9f01a0e0 message
+      @origin: mzyxwzks 9f01a0e0 message
+    "###);
+}
+
+fn get_branch_output(test_env: &TestEnvironment, repo_path: &Path) -> String {
+    test_env.jj_cmd_success(repo_path, &["branch", "list", "--all"])
+}

docs/design/tracking-branches.md

@@ -222,8 +222,7 @@ In particular, a merge of local and remote targets is
 * `jj git clone`
   * Import, track, and merge per `git.auto_local_branch` config.
   * The default branch will be tracked regardless of `git.auto_local_branch`
-    config. (Because local branch is created for the default remote branch,
-    it makes sense to track.)
+    config. This isn't technically needed, but will help users coming from Git.
 
 ### branch