This adds xz as a compression option.
xz is alread supported by bsdtar without the external xz program.
Upstream-commit: 3c5d2e4661e76e83e35637561153578ddd87f249
Component: engine
As per FIXME, CmdStream could have deadlocked if a command printed
enough on stderr. This commit fixes that, but still keeps all of
the stderr output in memory.
Upstream-commit: 58befe3081726ef74ea09198cd9488fb42c51f51
Component: engine
