Make post push receive message optional

Me and my users find the new gitlab_post_receive message about "Create merge request for ..." after pushing quite irritating. If you are used to have a very short push message you mistakenly now think "oh no, an error message, something went wrong ... no wait... hmm, no error ... ah, darn, that unnecessary message again!"

The message was introduced here, I think: 9ba4a0aa

Can we please get an option to turn that off?