filectx: use linkrev to sort ancestors

We're going to make rev() lazily do _adjustlinkrevs, and we don't want
that to happen when we're quickly tracing through file ancestry
without caring about revs (as we do when finding copies).

This takes us back to pre-linkrev-correction behavior, but shouldn't
regress us relative to the last stable release.

mercurial/context.py

@@ -954,7 +954,7 b' class basefilectx(object):'
         cut = followfirst and 1 or None
         while True:
             for parent in c.parents()[:cut]:
-                visit[(parent.rev(), parent.node())] = parent
+                visit[(parent.linkrev(), parent.filenode())] = parent
             if not visit:
                 break
             c = visit.pop(max(visit))