Use different exit code when desired dep roll is a no-op

roll_dep.py

 #!/usr/bin/env python
 # Copyright 2015 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.
 
 """Rolls DEPS controlled dependency.
 
 Works only with git checkout and git dependencies.  Currently this
 script will always roll to the tip of to origin/master.
 """
 
 import argparse
 import os
 import re
 import subprocess
 import sys
 
 NEED_SHELL = sys.platform.startswith('win')
 
 
 class Error(Exception):
   pass
 
 
+class AlreadyRolledError(Error):
+  pass
+
+
 def check_output(*args, **kwargs):
   """subprocess.check_output() passing shell=True on Windows for git."""
   kwargs.setdefault('shell', NEED_SHELL)
   return subprocess.check_output(*args, **kwargs)
 
 
 def check_call(*args, **kwargs):
   """subprocess.check_call() passing shell=True on Windows for git."""
   kwargs.setdefault('shell', NEED_SHELL)
   subprocess.check_call(*args, **kwargs)
@@ skipping 67 matching lines @@
     else:
       raise Error('Expected to find commit %s for %s in DEPS' % (head, key))
 
   print('Found old revision %s' % head)
 
   check_call(['git', 'fetch', 'origin', '--quiet'], cwd=full_dir)
   roll_to = check_output(['git', 'rev-parse', roll_to], cwd=full_dir).strip()
   print('Found new revision %s' % roll_to)
 
   if roll_to == head:
-    raise Error('No revision to roll!')
+    raise AlreadyRolledError('No revision to roll!')
 
   commit_range = '%s..%s' % (head[:9], roll_to[:9])
 
   upstream_url = check_output(
       ['git', 'config', 'remote.origin.url'], cwd=full_dir).strip()
   log_url = get_log_url(upstream_url, head, roll_to)
   cmd = [
     'git', 'log', commit_range, '--date=short', '--no-merges',
   ]
   logs = check_output(
@@ skipping 83 matching lines @@
         args.roll_to,
         args.key,
         reviewers,
         args.bug,
         args.no_log,
         args.log_limit,
         args.ignore_dirty_tree)
 
   except Error as e:
     sys.stderr.write('error: %s\n' % e)
-    return 1
+    return 2 if isinstance(e, AlreadyRolledError) else 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())