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 b877814e09b9f25308ec205cf48bb9554b33e95c)
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 e38231007e19e5f27b0e77e72bcd26fb3d76edfb)
Signed-off-by: Reinhard Tartler <siretart@tauware.de>