fail gracefully if git nav-downstream is called in cog

Bug: 339231299
Change-Id: If4d5dcc0ee0606acd8b539d19b29106f5152bba3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5540889
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
Auto-Submit: Yiwei Zhang <yiwzhang@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>

git_nav_downstream.py

@@ -11,13 +11,18 @@ which branch.
 import argparse
 import sys
 
-import gclient_utils
 from git_common import current_branch, branches, upstream, run, hash_one
+
+import gclient_utils
 import metrics
 
 
 @metrics.collector.collect_metrics('git nav-downstream')
 def main(args):
+    if gclient_utils.IsEnvCog():
+        print('nav-downstream command is not supported in non-git environment.',
+              file=sys.stderr)
+        return 1
     parser = argparse.ArgumentParser()
     parser.add_argument('--pick',
                         help=('The number to pick if this command would '