Treat all '*.pnm' files as non-text file

This convinces the pre-receive hook to not consider all *.pnm files as
text files to reduce the patch sizes and avoids triggering whitespace
checks,

Contains a correction by Janne Grunau <janne-libav@jannau.net>

(cherry picked from commit 0f257e29c5520b215e573e7e3abde8e5a4743309)
Signed-off-by: Reinhard Tartler <siretart@tauware.de>
Replace lena.pnm

The new reference.pnm is a freely licensed replacement. The photo has
been taken by Reinhard Tartler on August 28 2014, and is licensed under
the expat license as stated at http://www.jclark.com/xml/copying.txt

(cherry picked from commit 8895bf7b78650c0c21c88cec0484e138ec511a4b)
Signed-off-by: Reinhard Tartler <siretart@tauware.de>