Add the "message" subcommand to the gerrit command line tool

This allows placing a message into the gerrit CL with the "message"
command.

BUG=chromium:254515
TEST=From command line, try:
  gerrit message 32556 "testing 1, 2, 3. \" \\ '"
See this added to gerrit:
  testing 1, 2, 3. " \ '

Change-Id: I667ba1791f16b4ec29612ba3efb2a7dbc3cb22f2
Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/62776
Reviewed-by: David James <davidjames@chromium.org>
diff --git a/scripts/gerrit.py b/scripts/gerrit.py
index 92282bd..ac73a03 100644
--- a/scripts/gerrit.py
+++ b/scripts/gerrit.py
@@ -13,6 +13,7 @@
 
 import inspect
 import os
+import pipes
 import re
 
 from chromite.buildbot import constants
@@ -261,6 +262,11 @@
     opts.gerrit.SetReviewers(idx, add=add_list, remove=remove_list)
 
 
+def UserActMessage(opts, idx, message):
+  """Add a message to CL <n>"""
+  ReviewCommand(opts, idx, ['--message', pipes.quote(message)])
+
+
 def main(argv):
   # Locate actions that are exposed to the user.  All functions that start
   # with "UserAct" are fair game.