revision: make tree comparison functions take commits rather than trees

revision: make tree comparison functions take commits rather than trees

This will make it easier to do various clever things that don't depend
on the pure tree contents.  It also makes the parameter passing much
simpler - the callers doesn't really look at trees anywhere else, and
it's really the function that should look at the low-level details.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>