revlog: make isdescendantrev(a, b) check if a < b

Authored by martinvonz.

Description

revlog: make isdescendantrev(a, b) check if a < b

This check was taken from rebase.py. It seems to make sense to move it
here, but I haven't done any measurements.

Differential Revision: https://phab.mercurial-scm.org/D3930

Details