[GHF] Fix ghstack branches in sync logic (#93298)

Test plan:
```python
from git_utils import are_ghstack_branches_in_sync,GitRepo
repo=GitRepo("/Users/nshulga/git/pytorch/pytorch")
are_ghstack_branches_in_sync(repo, "gh/SS-JIA/206/head")
```

Pull Request resolved: https://github.com/pytorch/pytorch/pull/93298
Approved by: https://github.com/clee2000, https://github.com/ZainRizvi

.github/scripts/test_gitutils.py

@@ -1,6 +1,11 @@
 #!/usr/bin/env python3
-from gitutils import PeekableIterator, patterns_to_regex
-from unittest import TestCase, main
+from gitutils import PeekableIterator, patterns_to_regex, GitRepo, are_ghstack_branches_in_sync, _shasum
+from unittest import TestCase, main, SkipTest
+from pathlib import Path
+
+
+BASE_DIR = Path(__file__).parent
+
 
 class TestPeekableIterator(TestCase):
     def test_iterator(self, input_: str = "abcdef") -> None:
@@ -35,5 +40,34 @@ class TestPattern(TestCase):
             self.assertTrue(patterns_re.match(filename))
 
 
+class TestGitRepo(TestCase):
+    def setUp(self) -> None:
+        repo_dir = BASE_DIR.parent.parent.absolute()
+        if not (repo_dir / ".git").is_dir():
+            raise SkipTest("Can't find git directory, make sure to run this test on real repo checkout")
+        self.repo = GitRepo(str(repo_dir))
+
+    def _skip_if_ref_does_not_exist(self, ref: str) -> None:
+        """ Skip test if ref is missing as stale branches are deleted with time """
+        try:
+            self.repo.show_ref(ref)
+        except RuntimeError as e:
+            raise SkipTest(f"Can't find head ref {ref} due to {str(e)}") from e
+
+    def test_compute_diff(self) -> None:
+        diff = self.repo.diff("HEAD")
+        sha = _shasum(diff)
+        self.assertEqual(len(sha), 64)
+
+    def test_ghstack_branches_in_sync(self) -> None:
+        head_ref = "gh/SS-JIA/206/head"
+        self._skip_if_ref_does_not_exist(head_ref)
+        self.assertTrue(are_ghstack_branches_in_sync(self.repo, head_ref))
+
+    def test_ghstack_branches_not_in_sync(self) -> None:
+        head_ref = "gh/clee2000/1/head"
+        self._skip_if_ref_does_not_exist(head_ref)
+        self.assertFalse(are_ghstack_branches_in_sync(self.repo, head_ref))
+
 if __name__ == '__main__':
     main()