annotate: add core algorithm to skip a rev The core algorithm is inspired by git hyper-blame, implemented at https://chromium.googlesource.com/chromium/tools/depot_tools.git/+/master/git_hyper_blame.py. The heuristic is as documented in the comments.

annotate: make pair take all parents to pair against In upcoming patches we'll need to be aware of all parents at the same time. This also exposes a potential bug: if a line can be annotated with both parents of a merge commit, it'll always be annotated with p2, not p1. I'm not sure if that's what we want, but at least the code makes it clear now.