[PATCH 0 of 4 resend] faster ancestor patchs

[Bryan O'Sullivan]

These patches have been in production for almost a year at Facebook.
I originally held back on pushing them at Matt's request, because
there's some "generic ancestor" code in the merge module that he
wanted to excise. Unfortunately, we're not any closer to excising
that code, so it seems unwise to continue to sit on these patches.