refs: Add a --collections argument to the built-in refs command

refs: Add a --collections argument to the built-in refs command

Rather than change the output format used by the existing refs command
to output collection IDs in addition to ref names, this functionality has
been hidden behind an --collections argument. If it’s not specified `ostree
refs` will output the same content as before for a given repository. If
it is specified, the collection ID for each ref will be included in the
output as (collection ID, ref name).

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #924
Approved by: cgwalters