mkosi: add helper script to update mkosi hash

mkosi: add helper script to update mkosi hash

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.