fix: retain GIT_COMMON_DIR during discovery

Repository validation can use GIT_COMMON_DIR for refs and objects. Preserve the same value on Repo so later config, ref, and object access uses the directory that made discovery succeed.

This addresses the substantive review finding on a6a38a85 and adds focused coverage for environment-provided common storage.

Validation: 9 focused tests and 4 subtests; Ruff check/format; mypy; compileall; git diff --check.

Author: codex

git/repo/base.py

@@ -363,11 +363,15 @@ def __init__(
             # Let's not assume the option exists, although it should.
             pass
 
-        try:
-            common_dir = (Path(self.git_dir) / "commondir").read_text().splitlines()[0].strip()
-            self._common_dir = osp.join(self.git_dir, common_dir)
-        except OSError:
-            self._common_dir = ""
+        common_dir = os.getenv("GIT_COMMON_DIR")
+        if common_dir is not None:
+            self._common_dir = common_dir
+        else:
+            try:
+                common_dir = (Path(self.git_dir) / "commondir").read_text().splitlines()[0].strip()
+                self._common_dir = osp.join(self.git_dir, common_dir)
+            except OSError:
+                self._common_dir = ""
 
         # Adjust the working directory in case we are actually bare - we didn't know
         # that in the first place.

test/test_repo.py

@@ -162,6 +162,22 @@ def test_repo_discovery_rejects_invalid_metadata(self):
             with self.subTest(metadata=".git"):
                 self.assertRaises(InvalidGitRepositoryError, Repo, path)
 
+    def test_repo_discovery_uses_git_common_dir(self):
+        with tempfile.TemporaryDirectory() as tdir:
+            git_dir = Path(tdir) / "git"
+            common_dir = Path(tdir) / "common"
+            git_dir.mkdir()
+            common_dir.mkdir()
+            (git_dir / "HEAD").write_text("ref: refs/heads/main\n")
+            (common_dir / "objects").mkdir()
+            (common_dir / "refs").mkdir()
+
+            with mock.patch.dict(os.environ, {"GIT_DIR": str(git_dir), "GIT_COMMON_DIR": str(common_dir)}):
+                repo = Repo()
+
+            assert osp.samefile(repo.common_dir, common_dir)
+            assert osp.samefile(repo.odb.root_path(), common_dir / "objects")
+
     @with_rw_repo("0.3.2.1")
     def test_repo_creation_from_different_paths(self, rw_repo):
         r_from_gitdir = Repo(rw_repo.git_dir)