sync/git(fix[update_repo]): Return result on stash-save failure

why: Stash-save error handler was missing return statement, allowing
checkout/rebase to proceed with unstashed changes and stash.pop()
to fail on a non-existent stash.
what:
- Add return result after stash-save error recording

Author: tony

src/libvcs/sync/git.py

@@ -506,6 +506,7 @@ def update_repo(
                 except exc.CommandError as e:
                     self.log.exception("Failed to stash changes")
                     result.add_error("stash-save", str(e), exception=e)
+                    return result
 
             # Checkout the remote branch
             try: