Target branch of merge request should only be changed in open state

This happened in gitlab-com/www-gitlab-com!53383 (merged). The target branch was changed after the merge request was merged, which made it seem like the MR landed in the newly-assigned target branch. We should lock target branch changes unless the merge request is open. Why? If the merge request is closed, changing the target branch won't refresh the diff.

Steps to reproduce:

1. Merge a merge request to master.
2. Change the target branch to something else.
3. Notice that the Merged into shows the wrong branch