git: preserve HEAD when possible

Closes: #2210

cli/tests/test_git_colocated.rs

@@ -410,6 +410,68 @@ fn test_git_colocated_conflicting_git_refs() {
     "###);
 }
 
+#[test]
+fn test_git_colocated_checkout_non_empty_working_copy() {
+    let test_env = TestEnvironment::default();
+    let workspace_root = test_env.env_root().join("repo");
+    let git_repo = git2::Repository::init(&workspace_root).unwrap();
+    test_env.jj_cmd_ok(&workspace_root, &["init", "--git-repo", "."]);
+
+    // Create an initial commit in Git
+    // We use this to set HEAD to master
+    std::fs::write(workspace_root.join("file"), "contents").unwrap();
+    git_repo
+        .index()
+        .unwrap()
+        .add_path(Path::new("file"))
+        .unwrap();
+    let tree1_oid = git_repo.index().unwrap().write_tree().unwrap();
+    let tree1 = git_repo.find_tree(tree1_oid).unwrap();
+    let signature = git2::Signature::new(
+        "Someone",
+        "someone@example.com",
+        &git2::Time::new(1234567890, 60),
+    )
+    .unwrap();
+    git_repo
+        .commit(
+            Some("refs/heads/master"),
+            &signature,
+            &signature,
+            "initial",
+            &tree1,
+            &[],
+        )
+        .unwrap();
+    insta::assert_snapshot!(
+        git_repo.head().unwrap().peel_to_commit().unwrap().id().to_string(),
+        @"e61b6729ff4292870702f2f72b2a60165679ef37"
+    );
+
+    std::fs::write(workspace_root.join("two"), "y").unwrap();
+
+    test_env.jj_cmd_ok(&workspace_root, &["describe", "-m", "two"]);
+    test_env.jj_cmd_ok(&workspace_root, &["co", "@-"]);
+    let (_, stderr) = test_env.jj_cmd_ok(&workspace_root, &["describe", "-m", "new"]);
+    insta::assert_snapshot!(stderr, @r###"
+    Working copy now at: kkmpptxz 4c049607 (empty) new
+    Parent commit      : lnksqltp e61b6729 master | initial
+    "###);
+
+    let git_head = git_repo.find_reference("HEAD").unwrap();
+    let git_head_target = git_head.symbolic_target().unwrap();
+
+    assert_eq!(git_head_target, "refs/heads/master");
+
+    insta::assert_snapshot!(get_log_output(&test_env, &workspace_root), @r###"
+    @  4c04960765ca906d0cb25b15a946be4c0dd71b8e new
+    │ ◉  4ec6f6506bd1903410f15b80058a7f0d8f62deea two
+    ├─╯
+    ◉  e61b6729ff4292870702f2f72b2a60165679ef37 master HEAD@git initial
+    ◉  0000000000000000000000000000000000000000
+    "###);
+}
+
 #[test]
 fn test_git_colocated_fetch_deleted_or_moved_branch() {
     let test_env = TestEnvironment::default();

lib/src/git.rs

@@ -809,11 +809,15 @@ pub fn reset_head(
 ) -> Result<(), git2::Error> {
     let first_parent_id = &wc_commit.parent_ids()[0];
     if first_parent_id != mut_repo.store().root_commit_id() {
+        let first_parent = RefTarget::normal(first_parent_id.clone());
+        let git_head = mut_repo.view().git_head();
         let new_git_commit_id = Oid::from_bytes(first_parent_id.as_bytes()).unwrap();
         let new_git_commit = git_repo.find_commit(new_git_commit_id)?;
-        git_repo.set_head_detached(new_git_commit_id)?;
+        if git_head != &first_parent {
+            git_repo.set_head_detached(new_git_commit_id)?;
+            mut_repo.set_git_head_target(first_parent);
+        }
         git_repo.reset(new_git_commit.as_object(), git2::ResetType::Mixed, None)?;
-        mut_repo.set_git_head_target(RefTarget::normal(first_parent_id.clone()));
     } else {
         // Can't detach HEAD without a commit. Use placeholder ref to nullify the HEAD.
         // We can't set_head() an arbitrary unborn ref, so use reference_symbolic()