Check for keep-around commits once if target and source projects are the same

This reduces the number of RefExists RPC call.

app/models/merge_request_diff.rb

@@ -290,7 +290,7 @@ class MergeRequestDiff < ActiveRecord::Base
   end
 
   def keep_around_commits
-    [repository, merge_request.source_project.repository].each do |repo|
+    [repository, merge_request.source_project.repository].uniq.each do |repo|
       repo.keep_around(start_commit_sha)
       repo.keep_around(head_commit_sha)
       repo.keep_around(base_commit_sha)