context: add "descendant()" to changectx for efficient descendant examination This patch adds "descendant()", which uses "revlog.descendant()" for descendant examination, to changectx. This implementation is more efficient than "new in old.descendants()" expression, because: - "changectx.descendants()" creates temporary "changectx" objects, but "revlog.descendant()" doesn't "revlog.descendant()" checks only revision numbers of descendants. - "revlog.descendant()" stops scanning, when scanning of all revisions less than one of examination target is finished this can avoid useless scanning in "not descendant" case.