project: support reftable anchors in worktree .git migration

The reftable backend creates real refs/ and reftable/ dirs. Update
_MigrateOldWorkTreeGitDir to expect these dirs and remove them.

Bug: 476209856
Change-Id: I4700da70cb466e25ecbc51ba4de9a906b8716bd8
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/550761
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

tests/test_project.py

@@ -341,6 +341,7 @@ class MigrateWorkTreeTests(unittest.TestCase):
     """Check _MigrateOldWorkTreeGitDir handling."""
 
     _SYMLINKS = {
+        # go/keep-sorted start
         "config",
         "description",
         "hooks",
@@ -349,9 +350,11 @@ class MigrateWorkTreeTests(unittest.TestCase):
         "objects",
         "packed-refs",
         "refs",
+        "reftable",
         "rr-cache",
         "shallow",
         "svn",
+        # go/keep-sorted end
     }
     _FILES = {
         "COMMIT_EDITMSG",
@@ -430,6 +433,25 @@ class MigrateWorkTreeTests(unittest.TestCase):
             for name in self._SYMLINKS:
                 self.assertTrue((dotgit / name).is_symlink())
 
+    def test_reftable_anchor_with_refs_dir(self):
+        """Migrate when reftable/ and refs/ are directories."""
+        with self._simple_layout() as tempdir:
+            dotgit = tempdir / "src/test/.git"
+            (dotgit / "refs").unlink()
+            (dotgit / "refs").mkdir()
+            (dotgit / "refs" / "heads").write_text("dummy")
+
+            (dotgit / "reftable").unlink()
+            (dotgit / "reftable").mkdir()
+            (dotgit / "reftable" / "tables.list").write_text("dummy")
+            project.Project._MigrateOldWorkTreeGitDir(str(dotgit))
+
+            self.assertTrue(dotgit.is_symlink())
+            self.assertEqual(
+                os.readlink(dotgit),
+                os.path.normpath("../../.repo/projects/src/test.git"),
+            )
+
 
 class ManifestPropertiesFetchedCorrectly(unittest.TestCase):
     """Ensure properties are fetched properly."""