git_superproject: avoid re-initing bare repo

Running sync with reftable on a files-backed workspace fails to re-init
the superproject dir with:
```
fatal: could not open
'.../.repo/exp-superproject/<hash>-superproject.git/refs/heads' for writing:
Is a directory
```

Bug: 476209856
Change-Id: Ie8473d66069aafefa5661bd3ea8e73b2b27c6a38
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/550981
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>

tests/test_git_superproject.py

@@ -461,6 +461,62 @@ class SuperprojectTestCase(unittest.TestCase):
                         "</manifest>",
                     )
 
+    def test_Init_success(self):
+        """Test _Init succeeds and creates the work git dir."""
+        self.assertFalse(os.path.exists(self._superproject._work_git))
+        with mock.patch(
+            "git_superproject.GitCommand", autospec=True
+        ) as mock_git_command:
+            instance = mock_git_command.return_value
+            instance.Wait.return_value = 0
+
+            self.assertTrue(self._superproject._Init())
+
+            mock_git_command.assert_called_once()
+            args, kwargs = mock_git_command.call_args
+            self.assertEqual(args[1][:2], ["init", "--bare"])
+
+            tmp_git_name = args[1][2]
+            self.assertTrue(
+                tmp_git_name.startswith(".tmp-superproject-initgitdir-")
+            )
+            self.assertTrue(os.path.exists(self._superproject._work_git))
+
+            tmp_git_path = os.path.join(
+                self._superproject._superproject_path, tmp_git_name
+            )
+            self.assertFalse(os.path.exists(tmp_git_path))
+
+    def test_Init_already_exists(self):
+        """Test _Init returns early if the work git dir already exists."""
+        os.mkdir(self._superproject._superproject_path)
+        os.mkdir(self._superproject._work_git)
+        with mock.patch(
+            "git_superproject.GitCommand", autospec=True
+        ) as mock_git_command:
+            self.assertTrue(self._superproject._Init())
+            mock_git_command.assert_not_called()
+
+    def test_Init_failure_cleans_up(self):
+        """Test _Init cleans up the temporary directory if 'git init' fails."""
+        with mock.patch(
+            "git_superproject.GitCommand", autospec=True
+        ) as mock_git_command:
+            instance = mock_git_command.return_value
+            instance.Wait.return_value = 1
+            instance.stderr = "mock git init failure"
+
+            self.assertFalse(self._superproject._Init())
+            mock_git_command.assert_called_once()
+            args, kwargs = mock_git_command.call_args
+            tmp_git_name = args[1][2]
+
+            tmp_git_path = os.path.join(
+                self._superproject._superproject_path, tmp_git_name
+            )
+            self.assertFalse(os.path.exists(tmp_git_path))
+            self.assertFalse(os.path.exists(self._superproject._work_git))
+
     def test_Fetch(self):
         manifest = self.getXmlManifest(
             """