Improve MR code reloading when push code

Every time you pushed to master it updates merge requests that has master
as target branch. So if you have 50 open merge requests point to master
it will reload all of them every time you push a single commit to
master. The funny thing is that after reloading diff of most merge
requests looks the same.

After this patch we update diff only if we push commit to master that
includes in MR commits list.

For example we have next repository:

feature: A - B - C
master: A

We create merge requests #1 with code from feature to master.

MR #1: B - C

If we push to master commit D - MR will not be reloaded. So picture will
look next:

feature: A - B - C
master: A - D
MR #1: B - C

And if we push to master commit B - MR will be reloaded. So picture will
look next:

feature: A - B - C
master: A - B
MR #1: C
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/services/merge_requests/refresh_service.rb

@@ -43,8 +43,22 @@ module MergeRequests
       merge_requests = filter_merge_requests(merge_requests)
 
       merge_requests.each do |merge_request|
-        merge_request.reload_code
-        merge_request.mark_as_unchecked
+
+        if merge_request.source_branch == @branch_name
+          merge_request.reload_code
+          merge_request.mark_as_unchecked
+        else
+          mr_commit_ids = merge_request.commits.map(&:id)
+          push_commit_ids = @commits.map(&:id)
+          matches = mr_commit_ids & push_commit_ids
+
+          if matches.any?
+            merge_request.reload_code
+            merge_request.mark_as_unchecked
+          else
+            merge_request.mark_as_unchecked
+          end
+        end
       end
     end