Pull request version out of date error after a merge fails

Issue Summary

When a pull request merge fails (eg. through a hook veto or when merge fails using fast-forward only strategy) and the merge button is clicked again without closing the merge dialog, a pull request out-of-date error is shown which requires refreshing the page. 

Steps to Reproduce

1. Set up a pull request that will fail to merge using fast-forward only merge strategy
2. Click merge which will fail
3. Change strategy to merge commit
4. Click merge and pull request out of date error is displayed

Expected Results

The pull request should be merged.

Actual Results

The pull request is not merged and we get the attached error.

Workaround

The page needs to be refreshed and then merged.