repository: retrieve worktree HEAD via refdb

The function `git_repository_head_for_worktree` currently uses
`git_reference__read_head` to directly read a given worktree's HEAD from
the filesystem. This is broken in case the repository uses a different
refdb implementation than the filesystem-based one, so let's instead
open the worktree as a real repository and use `git_reference_lookup`.
This also fixes the case where the worktree's HEAD is not a symref, but
a detached HEAD, which would have resulted in an error previously.

src/repository.c

@@ -2177,12 +2177,6 @@ int git_repository_head_detached(git_repository *repo)
 	return exists;
 }
 
-static int get_worktree_file_path(git_buf *out, git_repository *repo, const char *worktree, const char *file)
-{
-	git_buf_clear(out);
-	return git_buf_printf(out, "%s/worktrees/%s/%s", repo->commondir, worktree, file);
-}
-
 int git_repository_head_detached_for_worktree(git_repository *repo, const char *name)
 {
 	git_reference *ref = NULL;
@@ -2223,7 +2217,8 @@ int git_repository_head(git_reference **head_out, git_repository *repo)
 
 int git_repository_head_for_worktree(git_reference **out, git_repository *repo, const char *name)
 {
-	git_buf path = GIT_BUF_INIT;
+	git_repository *worktree_repo = NULL;
+	git_worktree *worktree = NULL;
 	git_reference *head = NULL;
 	int error;
 
@@ -2231,26 +2226,23 @@ int git_repository_head_for_worktree(git_reference **out, git_repository *repo,
 
 	*out = NULL;
 
-	if ((error = get_worktree_file_path(&path, repo, name, GIT_HEAD_FILE)) < 0 ||
-	    (error = git_reference__read_head(&head, repo, path.ptr)) < 0)
+	if ((error = git_worktree_lookup(&worktree, repo, name)) < 0 ||
+	    (error = git_repository_open_from_worktree(&worktree_repo, worktree)) < 0 ||
+	    (error = git_reference_lookup(&head, worktree_repo, GIT_HEAD_FILE)) < 0)
 		goto out;
 
 	if (git_reference_type(head) != GIT_REFERENCE_DIRECT) {
-		git_reference *resolved;
-
-		error = git_reference_lookup_resolved(&resolved, repo, git_reference_symbolic_target(head), -1);
-		git_reference_free(head);
-		head = resolved;
-	}
-
+		if ((error = git_reference_lookup_resolved(out, worktree_repo, git_reference_symbolic_target(head), -1)) < 0)
+			goto out;
+	} else {
 		*out = head;
+		head = NULL;
+	}
 
 out:
-	if (error)
 	git_reference_free(head);
-
-	git_buf_dispose(&path);
-
+	git_worktree_free(worktree);
+	git_repository_free(worktree_repo);
 	return error;
 }
 

tests/worktree/repository.c

@@ -50,9 +50,12 @@ void test_worktree_repository__head_detached(void)
 
 	cl_assert(git_repository_head_detached(fixture.worktree));
 	cl_assert(git_repository_head_detached_for_worktree(fixture.repo, "testrepo-worktree"));
-	cl_git_fail(git_repository_head_for_worktree(&head, fixture.repo, "testrepo-worktree"));
+	cl_git_pass(git_repository_head_for_worktree(&head, fixture.repo, "testrepo-worktree"));
+
+	cl_assert_equal_oid(&ref->target.oid, &head->target.oid);
 
 	git_reference_free(ref);
+	git_reference_free(head);
 }
 
 void test_worktree_repository__head_detached_fails_for_invalid_worktree(void)