[recipes.py] use argparse to set command automatically.

recipes.py

 #!/usr/bin/env python
 # Copyright 2017 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 """Tool to interact with recipe repositories.
 
 This tool operates on the nearest ancestor directory containing an
 infra/config/recipes.cfg.
 """
@@ skipping 69 matching lines @@
   remote,
 ]
 
 
 def main():
   parser = argparse.ArgumentParser(
     description='Interact with the recipe system.')
 
   common_postprocess_func = common_args.add_common_args(parser)
 
-  subp = parser.add_subparsers()
+  subp = parser.add_subparsers(dest='command')
   for module in _SUBCOMMANDS:
     module.add_subparser(subp)
 
   args = parser.parse_args()
   common_postprocess_func(parser, args)
   args.postprocess_func(parser, args)
 
   # If we're bootstrapping, construct our bootstrap environment. If we're
   # using a custom deps path, install our enviornment there too.
   if args.use_bootstrap and not env.USING_BOOTSTRAP:
@@ skipping 62 matching lines @@
 
   if not isinstance(ret, int):
     if ret is None:
       ret = 0
     else:
       print >> sys.stderr, ret
       ret = 1
   sys.stdout.flush()
   sys.stderr.flush()
   os._exit(ret)