fix: keep bare-repository worktrees non-bare (#2223)

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Opening a linked worktree created from a bare repository read `core.bare` from the
common repository and discarded the worktree path.

Keep a discovered linked worktree non-bare when its administrative directory has
a `commondir` marker. The regression compares Git rev-parse behavior and verifies
Repo.bare and working_tree_dir.

Git baseline: 0bd5a6920d7c4238e0d90ddc0e7e08866e84a0f1;
environment.c:is_bare_repository() and setup.c:check_repository_format_gently().
Git 2.50.1 reports the linked checkout as non-bare.

Assisted-by: GPT 5.6
Co-authored-by: GPT 5.6 <codex@openai.com>

Author: Byron

git/repo/base.py

@@ -386,6 +386,10 @@ def __init__(
             # Let's not assume the option exists, although it should.
             pass
 
+        # A linked worktree is not bare even when its main repository is.
+        if self._bare and self._working_tree_dir and osp.isfile(osp.join(self.git_dir, "commondir")):
+            self._bare = False
+
         # Adjust the working directory in case we are actually bare - we didn't know
         # that in the first place.
         if self._bare:

test/test_repo.py

@@ -1459,6 +1459,23 @@ def test_git_work_tree_dotgit(self, rw_dir, use_relative_paths=False):
 
         self.assertIsInstance(repo.heads["aaaaaaaa"], Head)
 
+    @with_rw_directory
+    def test_git_work_tree_from_bare_repo(self, rw_dir):
+        if Git().version_info[:3] < (2, 5, 1):
+            pytest.skip("worktree feature unsupported, needs git 2.5.1 or later")
+
+        bare_repo = self.rorepo.clone(join_path_native(rw_dir, "bare_repo"), bare=True)
+        worktree_path = join_path_native(rw_dir, "worktree_repo")
+        if Git.is_cygwin():
+            worktree_path = cygpath(worktree_path)
+        bare_repo.git.worktree("add", "--detach", worktree_path)
+
+        repo = Repo(worktree_path)
+
+        assert Git(worktree_path).rev_parse("--is-bare-repository") == "false"
+        assert not repo.bare
+        assert osp.samefile(repo.working_tree_dir, worktree_path)
+
     def test_git_work_tree_dotgit_relative(self):
         """Check that we find .git as a worktree file containing a relative path
         and find the worktree based on it."""