Add comment to merge request when new push happens

It allows track when user added new commits to existing merge request

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Pluralize commit text for note when push to existing MR

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'mr-push-note' into 'master'

Add comment to merge request when new push happens

Fixes #1494

See merge request !1065
Merge branch 'master' of github.com:gitlabhq/gitlabhq