This compares the `MergeRequest#diff_head_sha` before squashing or
merging to a sha passed by the caller. If the `diff_head_sha` changed,
this means that the branch was updated since the merge was scheduled.

The `diff_head_sha` is the one passed to gitaly for merging or
squashing. So validating it right before performing the merge rather
than only in the controller would prevent the
`MergeRequests::RefreshService` from changing it between scheduling
the merge and actually merging.