merge: use scmutil.intrev() to sort ctx objects

This moves wctx to the last, but that shouldn't matter. Only the order of
stored revisions is important.

mercurial/merge.py

@@ -801,7 +801,7 b' def manifestmerge(repo, wctx, p2, pa, br'
 
     # manifests fetched in order are going to be faster, so prime the caches
     [x.manifest() for x in
-     sorted(wctx.parents() + [p2, pa], key=lambda x: x.rev() or -1)]
+     sorted(wctx.parents() + [p2, pa], key=scmutil.intrev)]
 
     if followcopies:
         ret = copies.mergecopies(repo, wctx, p2, pa)