Uncancel jobs on PRs when a hash is retested

[brianmoose]

The sequence is

1. Someone creates a PR.
2. They push a new commit while CIVET is building. So CIVET cancels the current jobs and starts testing the new commit.
3. The user then force pushes again back to the original commit. This cancels the current jobs. It doesn't create a new event because one with the same commit hash already exists. It would be nice though if it automatically uncancelled the jobs on the original commit and started testing (leaving previously tested jobs that finished alone).