Merge branch 'deleted-source-branch-comment' into 'master'

Explain "js: true" in "deleted_source_branch_spec.rb"

See merge request !7949

spec/features/merge_requests/deleted_source_branch_spec.rb

 require 'spec_helper'
 
+# This test serves as a regression test for a bug that caused an error
+# message to be shown by JavaScript when the source branch was deleted.
+# Please do not remove "js: true".
 describe 'Deleted source branch', feature: true, js: true do
   let(:user) { create(:user) }
   let(:merge_request) { create(:merge_request) }