Do not assume that every subproject needs to contain a diff.

trychange.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 """Client-side script to send a try job to the try server. It communicates to
 the try server by either writting to a svn repository or by directly connecting
 to the server by HTTP.
 """
 
@@ skipping 769 matching lines @@
       diff = GetMungedDiff('', urllib.urlopen(diff_url).readlines())
       options.diff = ''.join(diff[0])
       changed_files = diff[1]
     else:
       # Use this as the base.
       root = checkouts[0].checkout_root
       diffs = []
       for checkout in checkouts:
         raw_diff = checkout.GenerateDiff()
         if not raw_diff:
-          logging.error('Empty or non-existant diff, exiting.')
-          return 1
+          continue
         diff = raw_diff.splitlines(True)
         path_diff = gclient_utils.PathDifference(root, checkout.checkout_root)
         # Munge it.
         diffs.extend(GetMungedDiff(path_diff, diff)[0])
+      if not diffs:
+        logging.error('Empty or non-existant diff, exiting.')
+        return 1
       options.diff = ''.join(diffs)
 
     if not options.name:
       if options.issue:
         options.name = 'Issue %s' % options.issue
       else:
         options.name = 'Unnamed'
         print('Note: use --name NAME to change the try job name.')
 
     if not options.email:
@@ skipping 78 matching lines @@
     return 1
   except (gclient_utils.Error, subprocess2.CalledProcessError), e:
     print >> sys.stderr, e
     return 1
   return 0
 
 
 if __name__ == "__main__":
   fix_encoding.fix_encoding()
   sys.exit(TryChange(None, None, False))