Mike Hommey <mh@glandium.org> [Mon, 11 Jan 2016 22:00:07 -0500] rev 27711
mdiff: don't emit a diff header for empty trivial deltas
An empty trivial delta, coded as (0, 0, 0) makes the delta application
do nothing, but still takes 12 bytes, while skipping it altogether works
as much, without taking any space at all.