Hoff does not re-run tests unless there has been any other PRs merged

[AlexCELewis]

If there is a flakey test on merging you should close and reopen the PR then try to remerge the PR. This should re-run the tests and will do so if it re-joins a queue. However, if there have been no merges in-between the failed flakey test and the re-run then hoff won't try to re-run the tests because the hash of the branch is the same so running the tests again is not triggered.

The current way to get around this bug is to make a slight modification to a commit that was pushed (e.g the commit message or an empty fixup) then re-run the hoff merge.

Hoff should re-run the tests when attempting to re-merge and should not rely on other PRs being merged to change the hash.