bin/refs: Allow overwriting existing ref

bin/refs: Allow overwriting existing ref

Currently if you want to update a non-alias ref, you need to first check
if it exists and use either `ostree refs --create` or `ostree reset` as
appropriate. That's unnecessarily complicated and is much less
convenient than the old `write-refs` builtin that simply called
`ostree_repo_set_ref_immediate()` without any checks.

Add a `--force` option to be used with `--create` that does not raise an
error when the destination ref already exists.

Closes: #1870
Approved by: jlebon