Use adequate description for new MR.

app/controllers/projects/merge_requests_controller.rb

@@ -200,7 +200,8 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       source_branch: @merge_request.revert_branch_name,
       target_branch: @merge_request.target_branch,
       source_project_id: @merge_request.target_project_id,
-      target_project_id: @merge_request.target_project_id
+      target_project_id: @merge_request.target_project_id,
+      description: "Reverts #{@merge_request.to_reference}"
     }}
 
     redirect_to new_namespace_project_merge_request_url(@project.namespace, @project, url_params)

app/services/merge_requests/build_service.rb

@@ -56,7 +56,7 @@ module MergeRequests
       if commits && commits.count == 1
         commit = commits.first
         merge_request.title       = commit.title
-        merge_request.description = commit.description.try(:strip)
+        merge_request.description ||= commit.description.try(:strip)
       else
         merge_request.title = merge_request.source_branch.titleize.humanize
       end