diff -r afe0b48ef2d7 -r 4a987060d97e mercurial/changegroup.py
--- a/mercurial/changegroup.py	Thu Apr 03 12:59:12 2014 -0500
+++ b/mercurial/changegroup.py	Tue Apr 01 14:40:35 2014 -0700
@@ -490,3 +490,23 @@
     bundler = bundle10(repo, bundlecaps)
     return getsubset(repo, outgoing, bundler, source)
 
+def getbundle(repo, source, heads=None, common=None, bundlecaps=None):
+    """Like changegroupsubset, but returns the set difference between the
+    ancestors of heads and the ancestors common.
+
+    If heads is None, use the local heads. If common is None, use [nullid].
+
+    The nodes in common might not all be known locally due to the way the
+    current discovery protocol works.
+    """
+    cl = repo.changelog
+    if common:
+        hasnode = cl.hasnode
+        common = [n for n in common if hasnode(n)]
+    else:
+        common = [nullid]
+    if not heads:
+        heads = cl.heads()
+    outgoing = discovery.outgoing(cl, common, heads)
+    return getlocalbundle(repo, source, outgoing, bundlecaps=bundlecaps)
+