Re: make dist using git archive

Here is an updated version of this patch set.

I have removed the "dirty check" stuff.  It didn't really work well/was 
buggy under meson, and it failed mysteriously on the Linux CI tasks.  So 
let's just drop that functionality for now.

I have also added a more complete commit message and some more code 
comments.

I have extracted the freebsd CI script fix into a separate patch (0002). 
  I think this is useful even if we don't take the full CI patch (0003).

About the 0003 patch: It seems useful in principle to test these things 
continuously.  The dist script runs about 10 seconds in each task, and 
takes a bit of disk space for the artifacts.  I'm not sure to what 
degree this might bother someone.