#894065: zstd support

While this has been accepted upstream, it doesn't look like they'll make another release before Buster.

You've made two uploads after I filed this patch; I noticed the first but not the second, resulting in bloated uncompressed tarballs on my machines running unstable. Thus, giving you a nice git commit you can apply in one go would both prevent further mishaps for me (I really shouldn't have been this enthusiastic with using this feature yet), and let people use zstd tarballs in Buster (rationale in the bug report).

Similar patches have been merged in mc, dircolors and so on.