Fix bug where manually merged branches in a MR would end up with an empty diff

This bug manifested in 8.1 with the refactoring of `RefreshService`. Here's what happens:

1. User create a new branch `foo`.
2. User creates a merge request for `foo`.
3. User merges `foo` into `master` by hand.
4. `RefreshService` reloads the merge request. Since `master` is equivalent to `foo`, this results in an empty diff.
5. `RefreshService` then closes the MR.

This wasn't an issue when you use the normal "Accept Merge Request" flow because the act of clicking the button closes the merge request, so step 4 never happens.

Closes #3314

See merge request !1758