lib/repo-refs: Add a flag to exclude listing from refs/mirrors

lib/repo-refs: Add a flag to exclude listing from refs/mirrors

Currently the flag OSTREE_REPO_LIST_REFS_EXT_EXCLUDE_REMOTES for
ostree_repo_list_collection_refs() means that refs in refs/remotes/
should be excluded but refs in refs/mirrors/ should still be checked, in
addition to refs/heads/ which is always checked. However in some
situations you want to exclude both remote and mirrored refs and only
check local "owned" ones. So this
commit adds a new flag OSTREE_REPO_LIST_REFS_EXT_EXCLUDE_MIRRORS which
lets you exclude refs/mirrors/ from the listing.

This way we can avoid breaking API but still allow the listing of local
collection-refs.

The impetus for this change is that I'm changing Flatpak to make more
use of refs/mirrors, and we need a way to specify that a collection-ref
is local when using ostree_repo_resolve_collection_ref() in, for
example, the implementation of the repo command. The subsequent commit
will make the changes needed there.

Closes: #1825
Approved by: jlebon