With the latest mkosi it's possible for MinimumVersion= to be a git
commit so let's start making use of that. This will make mkosi fail
if it's executed within the systemd repository and the checked out
commit is too old.
Putting the mkosi commit sha in mkosi/mkosi.conf also allows retrieving
it without having the full source tree available.
We also make a bunch of improvements to the fetch-mkosi.py script.
This is very similar to tools/fetch-distro.py. The idea is that we extend the
commit to update the mkosi hash with a git log --pretty=oneline output, so that
the reader can know what changes were actually included.
The motivation is that I'm always wondering what changed in mkosi when I see a
commit updating the hash, and it's nicer to have this information shown
directly in the commit.
The script does _not_ pull changes from upstream, on the assumption that the
person doing the commit always has a fresh checkout and that they tested with
that checkout.