Make isAncestor work with revsets that aren't literal 12-digit hashes. Add existsAndIsAncestor.
Make isAncestor work with revsets that aren't literal 12-digit hashes. Add existsAndIsAncestor.

nth10sd/funfuzz,MozillaSecurity/funfuzz,MozillaSecurity/funfuzz,nth10sd/funfuzz,nth10sd/funfuzz,MozillaSecurity/funfuzz