ostree/fsck: Add --verify-back-refs option

ostree/fsck: Add --verify-back-refs option

This new option verifies that the refs listed in the ref-bindings for
each commit all point to that commit (i.e. there aren’t multiple commits
listing the same ref in their ref-bindings, and there aren’t any commits
with non-empty ref-bindings which aren’t pointed at by a ref).

This is useful when generating a new repository from scratch, but not
useful when adding new commits to an existing repository (since the old
commits will still, correctly, have ref-bindings from when the refs
pointed at them). That’s why it has to be enabled explicitly using
--verify-back-refs, rather than being on by default.

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