Dont allow merge without commit message

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/javascripts/merge_requests.js.coffee

@@ -24,6 +24,8 @@ class MergeRequest
 
     modal = $('#modal_merge_info').modal(show: false)
 
+    disableButtonIfEmptyField '#merge_commit_message', '.accept_merge_request'
+
   # Local jQuery finder
   $: (selector) ->
     this.$el.find(selector)