EE: Fetch the merged branches at once

EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14729

It's a trivial conflict that I just need to remove all the markers. I guess Git would try to collapse two end while we indeed want 2 end because we're adding a new method or so. I wonder if Git could improve in this regard somehow. I could see that the current behaviour makes sense for general diff, but it's not guessing correctly for source codes.