fail gracefully if git mark-merge-base is called in cog

Bug: 339231299
Change-Id: I882365cd75e5945ecbf01bdca5a2b67735eb58da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5540670
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Auto-Submit: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>

git_mark_merge_base.py

@@ -18,8 +18,14 @@ from subprocess2 import CalledProcessError
 from git_common import remove_merge_base, manual_merge_base, current_branch
 from git_common import get_or_create_merge_base, hash_one, upstream
 
+import gclient_utils
+
 
 def main(argv):
+    if gclient_utils.IsEnvCog():
+        print('mark-merge-base command is not supported in non-git '
+              'environment.', file=sys.stderr)
+        return 1
     parser = argparse.ArgumentParser(
         description=__doc__.strip().splitlines()[0],
         epilog=' '.join(__doc__.strip().splitlines()[1:]))