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.
278d5bfd7e