libxfs_apply: filterdiff can't handle /dev/null properly

libxfs_apply: filterdiff can't handle /dev/null properly

Because we are mangling the diff source/destination locations, we
have to add prefixes to them to get them to apply cleanly as -p1
patches. This is all fine until we create or remove a file and
the the src/dest is /dev/null. Applying a prefix here causes
the diff to be malformed and it won't apply.

Add another hack to work around this limitation of filterdiff when
reformatting the diff into readable format.

Signed-off-by: Dave Chinner <david@fromorbit.com>