Enable auto-rebasing of other PRs

[mhamann]

When merging a PR into a branch via GitHub, other PRs may become out of date if they also target that same branch.

In order to ensure all branches are up to date, enumerate open PRs that target the just updated branch, then rebase them and force push them back to the remote repo in the same branch as the PR was originally opened.

The rebase must be successful with zero conflicts prior to pushing updated code to the branch.