diff -r 76f6ae06ddf5 -r a5a06c9c7407 mercurial/mdiff.py
--- a/mercurial/mdiff.py	Fri Jan 23 17:01:39 2015 -0500
+++ b/mercurial/mdiff.py	Wed Jan 21 16:35:09 2015 -0500
@@ -367,6 +367,9 @@
 def trivialdiffheader(length):
     return struct.pack(">lll", 0, 0, length)
 
+def replacediffheader(oldlen, newlen):
+    return struct.pack(">lll", 0, oldlen, newlen)
+
 patches = mpatch.patches
 patchedsize = mpatch.patchedsize
 textdiff = bdiff.bdiff