remotefilelog: get linkrev using an unfiltered changelog

Authored by durin42.

Description

remotefilelog: get linkrev using an unfiltered changelog

We'd previously managed to dodge this bullet because we overrode the prune()
method in cgpacker, but now that method is inlined and we should just have a
correct linkrev() method instead.

Differential Revision: https://phab.mercurial-scm.org/D4683