Make `git cl description` work for Gerrit

gerrit_util.py

 # Copyright (c) 2013 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """
 Utilities for requesting information for a gerrit server via https.
 
 https://gerrit-review.googlesource.com/Documentation/rest-api.html
 """
 
@@ skipping 512 matching lines @@
 
 
 def SubmitChange(host, change, wait_for_merge=True):
   """Submits a gerrit change via Gerrit."""
   path = 'changes/%s/submit' % change
   body = {'wait_for_merge': wait_for_merge}
   conn = CreateHttpConn(host, path, reqtype='POST', body=body)
   return ReadHttpJsonResponse(conn, ignore_404=False)
 
 
+def SetCommitMessage(host, change, description):
+  """Updates a commit message."""
+  # First, edit the commit message in a draft.
+  path = 'changes/%s/edit:message' % change
+  body = {'message': description}
+  conn = CreateHttpConn(host, path, reqtype='PUT', body=body)
+  try:
+    ReadHttpResponse(conn, ignore_404=False)
+  except GerritError as e:
+    # On success, gerrit returns status 204; anything else is an error.
+    if e.http_status != 204:
+      raise
+  else:
+    raise GerritError(
+        'Unexpectedly received a 200 http status while editing message in '
+        'change %s' % change)
+
+  # And then publish it.
+  path = 'changes/%s/edit:publish' % change
+  conn = CreateHttpConn(host, path, reqtype='POST', body={})
+  try:
+    ReadHttpResponse(conn, ignore_404=False)
+  except GerritError as e:
+    # On success, gerrit returns status 204; anything else is an error.
+    if e.http_status != 204:
+      raise
+  else:
+    raise GerritError(
+        'Unexpectedly received a 200 http status while publishing message '
+        'change in %s' % change)
+
+
 def GetReviewers(host, change):
   """Get information about all reviewers attached to a change."""
   path = 'changes/%s/reviewers' % change
   return ReadHttpJsonResponse(CreateHttpConn(host, path))
 
 
 def GetReview(host, change, revision):
   """Get review information about a specific revision of a change."""
   path = 'changes/%s/revisions/%s/review' % (change, revision)
   return ReadHttpJsonResponse(CreateHttpConn(host, path))
@@ skipping 92 matching lines @@
         username = review.get('email', jmsg.get('name', ''))
         raise GerritError(200, 'Unable to set %s label for user "%s"'
                        ' on change %s.' % (label, username, change))
   jmsg = GetChangeCurrentRevision(host, change)
   if not jmsg:
     raise GerritError(
         200, 'Could not get review information for change "%s"' % change)
   elif jmsg[0]['current_revision'] != revision:
     raise GerritError(200, 'While resetting labels on change "%s", '
                    'a new patchset was uploaded.' % change)