fail gracefully if git reparent-branch is called in cog

Bug: 339231299
Change-Id: I395095bbdf4d097ff1ab236eb07cf308cef04799
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5540929
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
Auto-Submit: Yiwei Zhang <yiwzhang@google.com>

git_reparent_branch.py

@@ -13,12 +13,18 @@ from git_common import upstream, current_branch, run, tags, set_branch_config
 from git_common import get_or_create_merge_base, root, manual_merge_base
 from git_common import get_branch_tree, topo_iter
 
+import gclient_utils
 import git_rebase_update
 import metrics
 
 
 @metrics.collector.collect_metrics('git reparent-branch')
 def main(args):
+    if gclient_utils.IsEnvCog():
+        print(
+            'reparent-branch command is not supported in non-git environment.',
+            file=sys.stderr)
+        return 1
     root_ref = root()
 
     parser = argparse.ArgumentParser()