Reported by joe 23, Nov 27, 2010

In this case, diff should use the "from" filename not the 
"pseudo-to" filename /dev/null.

Steps to reproduce the problem:
-------------------------------

1. mtn drop foo.jpg
2. mtn diff


Expected result:
----------------
#
# old_revision [...]
#
# delete "foo.jpg"
#
============================================================
# foo.jpg is binary

Actual results:
---------------
#
# old_revision [...]
#
# delete "foo.jpg"
#
============================================================
# /dev/null is binary


Output of `mtn version --full`:
-------------------------------
0.99.1