Support force-with-lease pushes

[odrobnik]

Resolves #22.

Summary

• Add PushLease and a Repository.push(... forceWithLease:) overload for tracking-based and explicit-OID lease checks.
• Enforce the lease in libgit2's push negotiation callback on the actual push connection, then send the update as a force refspec only when the lease matches.
• Add an integration test that verifies a stale lease refuses to overwrite an advanced remote branch and a fresh expected OID succeeds.

Verification

• swift build --build-tests --scratch-path /Volumes/SSD/SwiftPM/GitKit-issue-22
• swift test --skip-build --scratch-path /Volumes/SSD/SwiftPM/GitKit-issue-22

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