--- a/mercurial/context.py	Wed Feb 23 13:21:55 2011 +0500
+++ b/mercurial/context.py	Wed Feb 23 17:27:05 2011 -0600
@@ -550,10 +550,14 @@
         return None
 
     def ancestors(self):
-        seen = set(str(self))
+        seen = set()
         visit = [self]
         while visit:
-            for parent in visit.pop(0).parents():
+            parents = visit.pop(0).parents()
+            if len(parents) > 1 and parents[1].rev() > parents[0].rev():
+                # make sure we return ancestors in reverse revision order
+                parents = reversed(parents)
+            for parent in parents:
                 s = str(parent)
                 if s not in seen:
                     visit.append(parent)

--- a/tests/test-rename-merge1.t	Wed Feb 23 13:21:55 2011 +0500
+++ b/tests/test-rename-merge1.t	Wed Feb 23 17:27:05 2011 -0600
@@ -99,3 +99,29 @@
    b3
    b4
   2 files updated, 0 files merged, 1 files removed, 0 files unresolved
+
+Check for issue2642
+
+  $ hg init t
+  $ cd t
+
+  $ echo c0 > f1
+  $ hg ci -Aqm0
+
+  $ hg up null -q
+  $ echo c1 > f1 # backport
+  $ hg ci -Aqm1
+  $ hg mv f1 f2
+  $ hg ci -qm2
+
+  $ hg up 0 -q
+  $ hg merge 1 -q --tool internal:local
+  $ hg ci -qm3
+
+  $ hg merge 2
+  merging f1 and f2 to f2
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+
+  $ cat f2
+  c0