revlog: replace descendant(b, a) by isdescendantrev(a, b) (API) The "is" is to match "isancestor" and to make it clear that it doesn't return a descendant. The "rev" is to make it clear that it's not about nodeids (unlike e.g. isancestor()). The argument order change is just seems more natural (and makes isancestor() less confusing). Differential Revision: https://phab.mercurial-scm.org/D3929