bsdiff-produced patch is included in xar archive, where it's compressed
with bzip2 a second time. Leaving the bsdiff output uncompressed makes
it possible to compress it better in xar (e.g. using LZMA2 or gzip,
either of which performs better on some files).
Changes header signature of bsdiff patch file to BSDIFN40 to clearly
mark the format as different.
Backward compatibility is preserved, bspatch can read both BSDIFF40 and