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>
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