Give 'gclient revert' a --merge option to match sync

This is a workaround for the fact that 'gclient revert' calls
'gclient update' with the same `options` object, despite not
actually having the same set of command line options.

R=iannucci@chromium.org, kevcheng@chromium.org
BUG=631220

Review-Url: https://codereview.chromium.org/2184493002

gclient.py

@@ -2154,6 +2154,7 @@ def CMDrevert(parser, args):
   options.force = True
   options.reset = False
   options.delete_unversioned_trees = False
+  options.merge = False
   client = GClient.LoadCurrentConfig(options)
   if not client:
     raise gclient_utils.Error('client not configured; see \'gclient config\'')