Address review feedback about empty repository paths

The commit review noted that treating every present GIT_DIR as explicit broke two documented cases: an empty GIT_DIR must fall back to current-directory discovery, while an empty Repo path must still use a nonempty GIT_DIR.

Base explicit-mode selection on the same truthiness rules used to choose epath. Cover both boundaries alongside the nested-.git regression.

Validation: focused test with both subtests; Ruff check and format; mypy; git diff --check.

Author: codex

git/repo/base.py

@@ -295,7 +295,7 @@ def __init__(
         # Walk up the path to find the `.git` dir.
         curpath = os.fspath(epath) if epath is not None else ""
         git_dir: Optional[str] = None
-        explicit_git_dir = path is None and git_dir_env is not None
+        explicit_git_dir = not path and bool(git_dir_env)
         while curpath:
             # ABOUT osp.NORMPATH
             # It's important to normalize the paths, as submodules will otherwise

test/test_repo.py

@@ -155,8 +155,15 @@ def test_repo_discovery_honors_explicit_git_dir(self):
             Repo.init(git_dir / ".git", bare=True).close()
 
             with mock.patch.dict(os.environ, {"GIT_DIR": os.fspath(git_dir)}):
+                for path in (None, ""):
+                    with Repo(path) as repo, self.subTest(path=path):
+                        assert osp.samefile(repo.git_dir, git_dir)
+
+            worktree = Path(tdir) / "worktree"
+            Repo.init(worktree).close()
+            with cwd(worktree), mock.patch.dict(os.environ, {"GIT_DIR": ""}):
                 with Repo() as repo:
-                    assert osp.samefile(repo.git_dir, git_dir)
+                    assert osp.samefile(repo.git_dir, worktree / ".git")
 
     def test_repo_discovery_rejects_invalid_metadata(self):
         with tempfile.TemporaryDirectory() as tdir: