Simulating tags

[Colin Walters <walters verbum org>]

OSTree doesn't have tags right now - we really should.  But
in the meantime, I hacked together this:

https://github.com/cgwalters/ostree-scripts/blob/master/ostree-write-ref

Which is obviously a very simple script, but does things like
ensure the ref is only written once if you specify --create, kind
of like O_EXCL, and similar to what `git tag` does by default.