Make `git cl description` work for Gerrit

gerrit_util.py

Index: gerrit_util.py
diff --git a/gerrit_util.py b/gerrit_util.py
index 53dc69c0c6d2806231e86f97e520faa7edb0e9ee..727626fe08c9cc3a40c6a456d96ec8abfeda190b 100755
--- a/gerrit_util.py
+++ b/gerrit_util.py
@@ -530,6 +530,38 @@ def SubmitChange(host, change, wait_for_merge=True):
   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