cli/rev-parse: Add `--single` option

cli/rev-parse: Add `--single` option

In the current "ostree native container" flow, we're inserting
a commit object into the repo but with no refs.

We have hacks in a few places to find the commit digest via e.g.
`find repo/objects -name *.commit` but that's a horrible hack.
Add `ostree rev-parse --single` which will print the single commit,
and error out if there is not exactly one commit.

Co-authored-by: Jonathan Lebon <jonathan@jlebon.com>