filectx: allow custom comparators We're going to introduce other sorts of filectxes very soon, and we'd like the cmp method to function properly (i.e. commutatively) for them. The only way to make that happen is for this cmp method to call into that specialized one if that defines a custom comparator.

debugmergestate: print out record type for files We're going to add a separate record type for change/delete conflicts soon. We need to make sure they get stored with the correct record type so that older versions of Mercurial correctly abort when they see change/delete records.