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>

diff --git a/.gitattributes b/.gitattributes
new file mode 100644 (file)
index 0000000..a900528
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1 @@
+*.pnm -diff -text