revlog: add optional stoprev arg to revlog.ancestors() This will be used as a step in removing reachable() in a future diff. Doing it now because bryano is in the process of rewriting ancestors in C. This depends on bryano's patch to replace *revs with revs in the declaration of revlog.ancestors.

--- a/tests/test-revlog-ancestry.py.out	Fri Jun 01 12:45:16 2012 -0700
+++ b/tests/test-revlog-ancestry.py.out	Fri Jun 01 15:44:13 2012 -0700
@@ -4,6 +4,8 @@
 3 4 2 1 0 
 Ancestors of 5 and 4
 4 2 0 
+Ancestors of 7, stop at 6
+6 
 
 Descendants of 5
 7 8