Add :current_id compare-and-swap option to ReferenceCollection#create

[JonJagger]

Proposed in #1013.

Why

Advancing a reference atomically -- update it to a new OID only if it still
points at an expected OID -- is the one ref operation (that I need) Rugged cannot not do
in-process. I have to shell out to git update-ref <ref> <new> <old>,
even when doing everything else (index build, Rugged::Commit.create, reads)
through Rugged. libgit2 already exposes git_reference_create_matching for
exactly this; this wires it up.

What

Adds a :current_id option to Rugged::ReferenceCollection#create. When
present, it dispatches to git_reference_create_matching (compare-and-swap)
instead of git_reference_create:

repo.references.create("refs/heads/main", new_oid,
                       force: true, current_id: base_oid)

Behaviour (all covered by tests):

• ref points at current_id -> updated, returns the new Rugged::Reference
• ref exists but points elsewhere -> no change, returns nil (matches
  merge_trees / rebase.commit, which return nil on GIT_EMERGECONFLICT /
  GIT_EAPPLIED)
• ref does not exist -> raises Rugged::ReferenceError
• :current_id with a symbolic target -> ArgumentError
• malformed :current_id -> ArgumentError

Notes

• :current_id is only consulted for direct (OID) references; existing
  behaviour is unchanged when it is absent.
• rdoc and CHANGELOG updated.

See #1013 for the API discussion -- happy to reshape (e.g. onto #update or a
dedicated method) if you'd prefer.