mercurial/ancestor.py

--- a/mercurial/ancestor.py	Sat Mar 15 10:02:31 2008 -0500
+++ b/mercurial/ancestor.py	Sat Mar 15 10:02:31 2008 -0500
@@ -97,6 +97,9 @@
     # We keep track of the number of revisions in the heap that
     # we may be interested in.  We stop walking the graph as soon
     # as this number reaches 0.
+    if a == b:
+        return [a]
+
     WHITE = 1
     BLACK = 2
     ALLCOLORS = WHITE | BLACK