Fix copy detection corner case

Fix copy detection corner case

We were aborting if the copied from rev was before the cut-off limit,
rather than the copied _to_ rev. The copied from rev can be
arbitrarily old.

--- a/mercurial/merge.py	Tue Dec 12 12:40:51 2006 -0800
+++ b/mercurial/merge.py	Tue Dec 12 17:52:33 2006 -0600
@@ -109,10 +109,10 @@
         visit = [fctx]
         while visit:
             fc = visit.pop()
+            if fc.path() != orig and fc.path() not in old:
+                old[fc.path()] = 1
             if fc.rev() < limit:
                 continue
-            if fc.path() != orig and fc.path() not in old:
-                old[fc.path()] = 1
             visit += fc.parents()
 
         old = old.keys()