Maybe bisect needs a flag like --start-with-merges to focus it's efforts on -no-ff merges?
Also, what if there was a tool on top of bisect that could better utilize GitHub PR JSON to target the search pattern? That could even save you some time in the case where you already have CI information attached to your PRs...
There was a patch submitted in 2011[1] to do roughly that, but it got diverted into a different purpose and then seems to have disappeared. It certainly seems like it must be possible to have a git-bisect that can deal with branches.
