summary: Implement a new, more efficient, summary format

This drops the deltas from the summary and uses the per-commit
metadata field to add the cache data to avoid the need for the separate
xa.cache and xa.sparse-cache indexes. This way we avoid repeating the
refs in multiple places.

Nothing uses this format yet, but we still pass make check if we enable
it.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f