Add Commit.is_shallow property; document stats() limitation at shallow boundary (#2167)

Accessing .stats on a commit at the boundary of a shallow clone raises
GitCommandError because the commit's parent SHA was never fetched. This
adds an is_shallow property to detect this case ahead of time by checking
the repository's shallow file, and documents the limitation on stats().

Co-authored-by: Claude <noreply@anthropic.com>

Author: harshitayadavv

AUTHORS

@@ -57,5 +57,6 @@ Contributors are:
 -Jonas Scharpf <jonas.scharpf _at_ checkmk.com>
 -Gordon Marx
 -Enji Cooper
+-Harshita Yadav <harshitayadav504 _at_ gmail.com>
 
 Portions derived from other open source works and are clearly marked.

git/objects/commit.py

@@ -374,6 +374,11 @@ def stats(self) -> Stats:
         """Create a git stat from changes between this commit and its first parent
         or from all changes done if this is the very first commit.
 
+        :note:
+            If this commit is at the boundary of a shallow clone, this will
+            raise :exc:`~git.exc.GitCommandError`, since the parent object
+            was never fetched and only exists as a reference on this commit.
+
         :return:
             :class:`Stats`
         """