git-squash-branch: handle empty squashes and dirty trees

git_squash_branch.py

 #!/usr/bin/env python
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import argparse
 import sys
 
-from git_common import squash_current_branch
+import git_common
 
 def main(args):
   parser = argparse.ArgumentParser()
   parser.add_argument(
       '-m', '--message', metavar='<msg>', default='git squash commit.',
       help='Use the given <msg> as the first line of the commit message.')
   opts = parser.parse_args(args)
-  squash_current_branch(opts.message)
+  if git_common.is_dirty_git_tree('squash-branch'):
+    return 1
+  git_common.squash_current_branch(opts.message)
   return 0
 
 
 if __name__ == '__main__':
   try:
     sys.exit(main(sys.argv[1:]))
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)