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.

mercurial/context.py

@@ -288,6 +288,10 b' class changectx(object):'
         n = self._repo.changelog.ancestor(self._node, n2)
         return changectx(self._repo, n)
 
+    def descendant(self, other):
+        """True if other is descendant of this changeset"""
+        return self._repo.changelog.descendant(self._rev, other._rev)
+
     def walk(self, match):
         fset = set(match.files())
         # for dirstate.walk, files=['.'] means "walk the whole tree".