--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -245,6 +245,9 @@ def patch(a, bin):
 def get_matching_blocks(a, b):
     return [(d[0], d[2], d[1] - d[0]) for d in bdiff.blocks(a, b)]
 
+def trivialdiffheader(length):
+    return struct.pack(">lll", 0, 0, length)
+
 patches = mpatch.patches
 patchedsize = mpatch.patchedsize
 textdiff = bdiff.bdiff