git: add test to diagnose worktree gitfiles on windows

We're getting paths like this, I think.

    gitdir: \\?\C:\Users\runneradmin\AppData\Local\Temp\jj-test-fy26a2\second

But I don't know what git does, and I don't have access to a windows machine.
Adding this so the CI windows build can tell us what's wrong.

cli/tests/test_git_colocated.rs

@@ -18,6 +18,7 @@ use std::process::Command;
 use assert_cmd::assert::OutputAssertExt;
 use git2::Oid;
 
+use crate::common::get_stdout_string;
 use crate::common::TestEnvironment;
 
 #[test]
@@ -931,20 +932,6 @@ fn test_colocated_workspace_independent_heads() {
             initial_commit
         );
     }
-
-    // Now that the functionality is tested, check we wrote a nice .git file
-    let dot_git = std::fs::read_to_string(second_path.join(".git")).expect("reading .git failed");
-    assert_eq!(
-        dot_git,
-        format!(
-            "gitdir: {}\n",
-            repo_path
-                .join(".git")
-                .join("worktrees")
-                .join("second")
-                .display()
-        )
-    );
 }
 
 #[test]
@@ -1105,3 +1092,67 @@ fn test_colocated_workspace_forget_locked() {
 
     let _ = test_env.jj_cmd_ok(&repo_path, &["workspace", "forget", "second"]);
 }
+
+#[test]
+fn test_worktrees_match_git_behaviour() {
+    // TODO: Better way to disable the test if git command couldn't be executed
+    if Command::new("git").arg("--version").status().is_err() {
+        eprintln!("Skipping because git command might fail to run");
+        return;
+    }
+
+    let test_env = TestEnvironment::default();
+    let repo_path = test_env.env_root().join("repo");
+    let abc_path = test_env.env_root().join("abc");
+
+    let git_cmd_success = |test_env: &TestEnvironment, path: &Path, args: &[&str]| -> String {
+        let cmd = Command::new("git")
+            .args(args)
+            .current_dir(path)
+            .env("GIT_AUTHOR_NAME", "JJ Test Suite")
+            .env("GIT_AUTHOR_EMAIL", "[email protected]")
+            .env("GIT_AUTHOR_DATE", "2005-04-07T22:13:13Z")
+            .env("GIT_COMMITTER_NAME", "JJ Test Suite")
+            .env("GIT_COMMITTER_EMAIL", "[email protected]")
+            .env("GIT_COMMITTER_DATE", "2005-04-07T22:13:13Z")
+            .assert()
+            .success();
+        let stdout = get_stdout_string(&cmd);
+        test_env.normalize_output(&stdout)
+    };
+
+    let read_worktree_setup = || {
+        let gitfile = std::fs::read_to_string(abc_path.join(".git")).unwrap();
+        let worktree_path = repo_path.join(".git").join("worktrees").join("abc");
+        let gitdir = std::fs::read_to_string(worktree_path.join("gitdir")).unwrap();
+        let commondir = std::fs::read_to_string(worktree_path.join("commondir")).unwrap();
+        eprintln!("   abc/.git -> {gitfile}");
+        eprintln!("   repo/.git/worktrees/abc/gitdir: {gitdir}");
+        eprintln!("   repo/.git/worktrees/abc/commondir: {commondir}");
+        (gitfile, gitdir, commondir)
+    };
+
+    git_cmd_success(&test_env, test_env.env_root(), &["init", "repo"]);
+    std::fs::write(repo_path.join("file.txt"), "contents").unwrap();
+    git_cmd_success(&test_env, &repo_path, &["add", "file.txt"]);
+    git_cmd_success(&test_env, &repo_path, &["commit", "-m", "initial commit"]);
+    git_cmd_success(&test_env, &repo_path, &["worktree", "add", "../abc"]);
+
+    eprintln!("git:");
+    let (git_gitfile, git_gitdir, git_commondir) = read_worktree_setup();
+
+    std::fs::remove_dir_all(&repo_path).unwrap();
+    std::fs::remove_dir_all(&abc_path).unwrap();
+
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "--colocate", "repo"]);
+    std::fs::write(repo_path.join("file.txt"), "contents").unwrap();
+    test_env.jj_cmd_ok(&repo_path, &["commit", "-m", "initial commit"]);
+    test_env.jj_cmd_ok(&repo_path, &["workspace", "add", "--colocate", "../abc"]);
+
+    eprintln!("jj:");
+    let (jj_gitfile, jj_gitdir, jj_commondir) = read_worktree_setup();
+
+    assert_eq!(jj_gitfile, git_gitfile);
+    assert_eq!(jj_gitdir, git_gitdir);
+    assert_eq!(jj_commondir, git_commondir);
+}