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
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