Remove source branch button should be disable if there are new changes on the branch

We want to prevent this from happening:

- create + merge MR on branch `humpf`
- create MR from same branch, new commit on HEAD
- ‘remove source branch’ on first MR
- no more branch in second MR

cc @dzaporozhets @DouweM

• A creates MR1 with branch X
• B merges MR1 with branch X
• A pushes to branch X
• A creates MR2 with branch X
• B presses "Remove source branch" on MR1, thinking it's safe
• MR2 is suddenly without source branch

UI should not show "Remove source branch" when branch was pushed to after merge (i.e. branch HEAD != last MR commit)

mentioned in merge request !2701 (merged)

Status changed to closed by merge request !2701 (merged)

mentioned in commit c6e0228c

mentioned in commit b5befc73