NXD: Use merge description message as merge message by default

Include Merge description message in merge by default. User can disable description in merge message by clicking on `Don't include description in commit message`.

Drop the merge title which is "Merge branch SOURCE into TARGET" and only keep the MR title as commit message.

app/models/merge_request.rb

@@ -1301,7 +1301,8 @@ class MergeRequest < ApplicationRecord
     end
 
     message = [
-      "Merge branch '#{source_branch}' into '#{target_branch}'",
+      # NXD: We only want to keep the original MR title
+      #"Merge branch '#{source_branch}' into '#{target_branch}'",
       title
     ]
 

app/views/shared/_commit_message_container.html.haml

@@ -7,7 +7,7 @@
     .commit-message-container
       .max-width-marker
       = text_area_tag 'commit_message',
-        (params[:commit_message] || local_assigns[:text] || local_assigns[:placeholder]),
+        (local_assigns[:message_with_description] || params[:commit_message] || local_assigns[:text] || local_assigns[:placeholder]),
         class: 'form-control gl-form-input js-commit-message',
         placeholder: local_assigns[:placeholder],
         data: descriptions,