Resolve merge conflicts in an editor

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5479 allows picking ours / theirs, but a lot of cases need an editor. Even the example case in this screenshot does:

In this case, I want to keep both the rename regex -> regexp, and the change from /.../ to %r{...} regexes. If we have an editor, then this becomes tractable.