When handling backouts we try to extract a list of all the bugs/commits that they touch, lookup the sync for each commit, and apply the backout to that commit. It seems that once we've applied the backout to a sync we won't apply it to further syncs.

This is rather broken if a backout touches multiple wpt bugs. Instead of directly applying the actual backout, if we have a set of commits that were backed out, potentially belonging to multiple bugs/syncs, for handling the backout we should construct a corresponding set of web-platform-tests commits as reverts for the original commits, rather than just from applying the backout commit itself.