Add versioned storage methods for incremental export support:
- GetChangesSinceExport: returns changes since a commit hash, with
NeedsFullExport flag for invalid/GC'd commits
- CommitExists: checks if a commit hash exists, supports short prefixes
Also fixes dolt_diff syntax for embedded driver (from_ref, to_ref, table).
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>