--- a/mercurial/revlog.py	Fri Jun 01 12:45:16 2012 -0700
+++ b/mercurial/revlog.py	Fri Jun 01 15:44:13 2012 -0700
@@ -381,8 +381,9 @@
                     visit.append(p)
         return reachable
 
-    def ancestors(self, revs):
+    def ancestors(self, revs, stoprev=0):
         """Generate the ancestors of 'revs' in reverse topological order.
+        Does not generate revs lower than stoprev.
 
         Yield a sequence of revision numbers starting with the parents
         of each revision in revs, i.e., each revision is *not* considered
@@ -393,6 +394,8 @@
         seen = set([nullrev])
         while visit:
             for parent in self.parentrevs(visit.popleft()):
+                if parent < stoprev:
+                    continue
                 if parent not in seen:
                     visit.append(parent)
                     seen.add(parent)

--- a/tests/test-revlog-ancestry.py	Fri Jun 01 12:45:16 2012 -0700
+++ b/tests/test-revlog-ancestry.py	Fri Jun 01 15:44:13 2012 -0700
@@ -58,6 +58,10 @@
     for r in repo.changelog.ancestors([5, 4]):
         print r,
 
+    print '\nAncestors of 7, stop at 6'
+    for r in repo.changelog.ancestors([7], 6):
+        print r,
+
     # Descendants
     print '\n\nDescendants of 5'
     for r in repo.changelog.descendants([5]):

--- 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