dagwalker: drop a useless intermediate variable

dagwalker: drop a useless intermediate variable

The variable used to contain revs.set() but as the `.set()` has been removed
it is now useless.

--- a/mercurial/graphmod.py	Sat Oct 11 01:39:20 2014 -0700
+++ b/mercurial/graphmod.py	Wed Oct 08 02:43:51 2014 -0700
@@ -37,11 +37,10 @@
     lowestrev = revs.min()
     gpcache = {}
 
-    knownrevs = revs
     for rev in revs:
         ctx = repo[rev]
         parents = sorted(set([p.rev() for p in ctx.parents()
-                              if p.rev() in knownrevs]))
+                              if p.rev() in revs]))
         mpars = [p.rev() for p in ctx.parents() if
                  p.rev() != nullrev and p.rev() not in parents]