Disable merge and approve buttons in merge request widget if diff_head_sha changes

After making the merge request widget update automatically at regular polling intervals, we opened ourselves up to the following scenario:

1. MR reviewer takes a look at a merge request diff and decides it looks ok to merge
2. user updates the branch with new changes
3. MR reviewer's MR widget is updated to use the new SHA of the changes
4. MR reviewer clicks the merge button and unknowingly merges the latest version which he/she did not yet review.