When a forked merge request was created with the same branch name as the
target name, MergeRequests::RefreshService would always create a new
diff even though nothing had changed. For example, on GitLab.com:

1. There were a number of merge requests in the gitlab-ce and www-gitlab-com
projects that had old merge requests from the community.
2. These merge requests originated from forked projects and used the
source branch master.
3. When someone pushed to master in the main repository, MergeRequests::RefreshService
would see that master matched the merge requests in question and generated a new
diff.
4. This led to an explosion of merge request diffs and slowed down the "Changes"
tab considerably.

This change alters MergeRequests::RefreshService so that it will only
refresh the diff if the merge request's source project and branch
match. Otherwise, the refresh will only happen if a pushed commit
contains a commit relevant to the existing merge request.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53153