--- a/mercurial/cmdutil.py	Wed Dec 01 19:20:16 2010 +0100
+++ b/mercurial/cmdutil.py	Wed Dec 01 21:46:08 2010 +0100
@@ -699,6 +699,8 @@
         for chunk in patch.diff(repo, prev, node, opts=opts):
             fp.write(chunk)
 
+        fp.flush()
+
     for seqno, rev in enumerate(revs):
         single(rev, seqno + 1, fp)