reset S-waiting-on-author tag when new commits are pushed

[nikomatsakis]

I would like to be able to aggressively set the status of PRs to S-waiting-on-author and then filter those out of my display. For me to feel comfortable doing that, though, I need to be sure that I won't miss updates from them -- therefore, I would like highfive to reset the status back to S-waiting-on-review as soon as a new commit or comment is made. (Possibly just a new comment by the PR author? But probably any comment is fine -- or any comment not by bors.)