When use change branches link at MR form - save source branch selection instead of target one

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

CHANGELOG

@@ -20,7 +20,7 @@ v 7.11.0 (unreleased)
   - Add "Reply quoting selected text" shortcut key (`r`)
   - Fix bug causing `@whatever` inside an issue's first code block to be picked up as a user mention.
   - Fix bug causing `@whatever` inside an inline code snippet (backtick-style) to be picked up as a user mention.
-  -
+  - When use change branches link at MR form - save source branch selection instead of target one
   - Show Atom feed buttons everywhere where applicable.
   - Add project activity atom feed.
   - Don't crash when an MR from a fork has a cross-reference comment from the target project on one of its commits.

app/helpers/merge_requests_helper.rb

@@ -49,4 +49,16 @@ module MergeRequestsHelper
   def issues_sentence(issues)
     issues.map { |i| "##{i.iid}" }.to_sentence
   end
+
+  def mr_change_branches_path(merge_request)
+    new_namespace_project_merge_request_path(
+      @project.namespace, @project,
+      merge_request: {
+        source_project_id: @merge_request.source_project_id,
+        target_project_id: @merge_request.target_project_id,
+        source_branch: @merge_request.source_branch,
+        target_branch: nil
+      }
+    )
+  end
 end

app/views/projects/merge_requests/_new_submit.html.haml

@@ -7,7 +7,7 @@
   %strong.label-branch #{@merge_request.target_project_namespace}:#{@merge_request.target_branch}
 
   %span.pull-right
-    = link_to 'Change branches', new_namespace_project_merge_request_path(@project.namespace, @project)
+    = link_to 'Change branches', mr_change_branches_path(@merge_request)
 
 = form_for [@project.namespace.becomes(Namespace), @project, @merge_request], html: { class: "merge-request-form form-horizontal gfm-form" } do |f|
   .merge-request-form-info