bin/prune: Add --only-branch

bin/prune: Add --only-branch

In 5c940987e768523ef1411b65bcaad09fba6befef / #646 we
added `--retain-branch-depth`; this adds a symmetric
`--only-branch` for the case where a repo owner just
wants to prune a specific branch.

The implementation here is pretty straightforward; we
just walk all refs and inject the equivalent of
`--retain-branch-depth=$ref=-1` if they're *not* in
`--only-branch`.

Closes: https://github.com/ostreedev/ostree/issues/1115
Closes: #1127
Approved by: jlebon