annotate: make pair take all parents to pair against...
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.