Make it work for out-of-tree builds without requiring one to specify
the location of the xz executable.
Add xz --filters-help.
Make the output shorter by reducing the number of xz -lvv test files.
Show the value of LANGUAGE environment variable.
Show the xz.git version using git describe --abbrev=8 instead of =4.
Since 5.3.5beta, "xz --lzma2=mf=bt4,nice=2" works even though bt4 needs
at least nice=4. It is rounded up internally by liblzma when needed.
Fixes: 5cd9f0df78cc4f8a7807bf6104adea13034fbb45
