fail gracefully if git squash-branch is called in cog

Bug: 339231299
Change-Id: If343481d12cd3763897b8fc695d3607cc4d7f64f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5540931
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Auto-Submit: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>

git_squash_branch.py

@@ -6,10 +6,15 @@
 import argparse
 import sys
 
+import gclient_utils
 import git_common
 
 
 def main(args):
+    if gclient_utils.IsEnvCog():
+        print('squash-branch command is not supported in non-git environment.',
+              file=sys.stderr)
+        return 1
     parser = argparse.ArgumentParser()
     parser.add_argument(
         '-m',