sync/git(fix[update_repo]): Return early on invalid-upstream rebase

why: The invalid_upstream handler recorded the error (ok=False) but
fell through to stash pop, creating inconsistent state.
what:
- Add return result after add_error in the invalid_upstream branch

Author: tony

src/libvcs/sync/git.py

@@ -576,6 +576,7 @@ def update_repo(
                 if any(msg in str(e) for msg in ["invalid_upstream", "Aborting"]):
                     self.log.exception("Invalid upstream remote. Rebase aborted.")
                     result.add_error("rebase", str(e), exception=e)
+                    return result
                 else:
                     # Rebase failed: Restore previous state.
                     with contextlib.suppress(exc.CommandError):