ostree/commit: Allow --orphan and --bind-ref to be specified together

ostree/commit: Allow --orphan and --bind-ref to be specified together

Typically you’d use --branch and --bind-ref together to add additional
bindings as well as creating a main --branch for the commit. However,
you might also want to occasionally use --orphan --bind-ref to create a
commit with bindings for one or more refs, but not actually create any
of those refs pointing to the commit (you might create them as a later
step).

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #1347
Approved by: cgwalters