context: optimize linkrev adjustment in blockancestors() (issue5538) We set parent._descendantrev = child.rev() when walking parents in blockancestors() so that, when linkrev adjustment is perform for these, it starts from a close descendant instead of possibly topmost introrev. (See `self._adjustlinkrev(self._descendantrev)` in filectx._changeid().) This is similar to changeset c82d88dfaf59, which added a "f._changeid" instruction in annotate() for the same purpose. However, here, we set _descendantrev explicitly instead of relying on the '_changeid' cached property being accessed (with effect to set _changeid attribute) so that, in _parentfilectx() (called from parents()), we go through `if '_changeid' in vars(self) [...]` branch in which instruction `fctx._descendantrev = self.rev()` finally appears and does what we want. With this, we can roughly get a 3x speedup (including in example of issue5538 from mozilla-central repository) on usage of followlines revset (and equivalent hgweb request).