[Toybox] Going through Oliver's xz commit list...

[Rob Landley]

On Feb 23, 2024, Oliver Webb triaged the xz-embedded commit list since 
our version forked off from the upstream public domain one, and highlighted:

 > commit 89094f05f02b Adds the ARM64 BCJ encoder
 > commit 8f3ed8b1759a Adds a MicroLZMA decoder
 > commit 6f0e0c41e368 Add xz_dec_catrun() to support concatenated files

I applied the first one (but don't have a test file for it, maybe 
compressing an arm64 binary with debian's xz would exercise that 
codepath), and am applying the third now, but the second one is tricksy 
because it just adds a new function to library exports. The extract 
doesn't actually seem to _use_ it.

Is there a later commit? Is there a separate microlzma format I should 
add an lzmacat for or something...? (Or does a properly fiddled with 
xzcat detect this other format and extract it?)

Linux 6.15's initramfs compression options are gzip, bzip2, LZMA, XZ, 
LZO, LZ4, and ZSTD. We have three of those. I have ZSTD as a distant 
todo to look at but that's down after "screen" and "rsync".

I've always been a bit unclear on the differences between LZMA, XZ, LZO, 
and LZ4, and... is this a fifth one? If so what tar.blah extension it 
would be? (It's says it came out of the guts of a filesystem, so... 
great? But I'm not sure what to do with that information. Speaking of 
which, zstd would also come after both "mksquashfs" and "zip" unless I 
start seeing source tarballs only available compressed with it.)

Rob