Merge branch 'commit-message-prefilled' into 'master'

Prefills commit message in edit file

See #14488 

![Screen_Shot_2016-04-06_at_10.18.06](/uploads/2ace2a21a6ead8d83c79ef9ce5dd34cc/Screen_Shot_2016-04-06_at_10.18.06.png)

See merge request !3564

app/views/shared/_commit_message_container.html.haml

@@ -6,7 +6,7 @@
     .commit-message-container
       .max-width-marker
       = text_area_tag 'commit_message',
-          (params[:commit_message] || local_assigns[:text]),
+          (params[:commit_message] || local_assigns[:text] || local_assigns[:placeholder]),
           class: 'form-control js-commit-message', placeholder: local_assigns[:placeholder],
           required: true, rows: (local_assigns[:rows] || 3),
           id: "commit_message-#{nonce}"