Expose libgit2's git_reference_create_matching (atomic compare-and-swap ref update)

[JonJagger]

Motivation

I'd like to advance a reference atomically from in-process Rugged, the way
git update-ref <ref> <new-oid> <old-oid> does on the command line: update the
ref to a new OID only if it still points at the OID I expect. This is the one
operation that currently forces a service doing everything else in-process
(index build, Rugged::Commit.create, reads) to shell out to git.

libgit2 has the primitive -- git_reference_create_matching, which takes a
current_id and returns GIT_EMODIFIED if the ref has moved -- but Rugged
doesn't currently surface it. ReferenceCollection#create calls plain
git_reference_create, and #update uses git_reference_set_target; neither
lets the caller pass an expected current value.

Proposed API

Add a :current_id option to ReferenceCollection#create, dispatching to
git_reference_create_matching when present:

# advance refs/heads/main to new_oid only if it still points at base_oid
repo.references.create("refs/heads/main", new_oid,
                       force: true, current_id: base_oid)

Outcomes:

• ref points at current_id -> updated, returns the new Rugged::Reference
• ref exists but points elsewhere -> no change, returns nil (this mirrors how
  merge_trees returns nil on GIT_EMERGECONFLICT and rebase.commit on
  GIT_EAPPLIED -- a CAS miss is an expected outcome under concurrency, not an
  error)
• ref does not exist -> raises Rugged::ReferenceError (libgit2's not-found)
• :current_id with a symbolic target -> ArgumentError (CAS is OID-only)

Open question

Is create the right home for this, or would you prefer it on #update, or a
dedicated method? I put it on create because that mirrors libgit2's own
naming (..._create_matching) and handles both create-if-absent and
conditional-update in one call, but I'm happy to reshape it to whatever fits
Rugged's conventions best.

Status

I'm opening a PR alongside this issue (thin C wrapper + tests covering all the
outcomes above + CHANGELOG/rdoc). Happy to rework the API shape before it's
merged if you'd prefer a different form.