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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>