Do not enforce HEAD when a revision is not specified.

rietveld.py

 # coding: utf-8
 # 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.
 """Defines class Rietveld to easily access a rietveld instance.
 
 Security implications:
 
 The following hypothesis are made:
 - Rietveld enforces:
@@ skipping 297 matching lines @@
       self, issue, patchset, reason, clobber, revision, builders_and_tests):
     """Requests new try jobs.
 
     |builders_and_tests| is a map of builders: [tests] to run.
 
     Returns the keys of the new TryJobResult entites.
     """
     params = [
       ('reason', reason),
       ('clobber', 'True' if clobber else 'False'),
-      ('revision', revision if revision else 'HEAD'),
       ('builders', json.dumps(builders_and_tests)),
       ('xsrf_token', self.xsrf_token()),
     ]
+    if revision:
+      params.append(('revision', revision))
     return self.post('/%d/try/%d' % (issue, patchset), params)
 
   def get_pending_try_jobs(self, cursor=None, limit=100):
     """Retrieves the try job requests in pending state.
 
     Returns a tuple of the list of try jobs and the cursor for the next request.
     """
     url = '/get_pending_try_patchsets?limit=%d' % limit
     extra = ('&cursor=' + cursor) if cursor else ''
     data = json.loads(self.get(url + extra))
@@ skipping 41 matching lines @@
           if not 'Name or service not known' in e.reason:
             # Usually internal GAE flakiness.
             raise
         # If reaching this line, loop again. Uses a small backoff.
         time.sleep(1+maxtries*2)
     finally:
       upload.ErrorExit = old_error_exit
 
   # DEPRECATED.
   Send = get