detect delta cycles in resolve_object

[netliomax25-code]

1. Pack.resolve_object walks a pack object's delta chain, but the only cycle guard rejects a single self-referencing REF_DELTA (base_offset == prev_offset), so a pack holding two or more REF_DELTA objects that name each other is not caught.
2. get_raw drives that walk, so reading such an object never returns: each iteration re-inflates a delta and appends to delta_stack, so memory grows without bound. A pack file is untrusted input (a cloned or downloaded repository, a submodule, a pack read from disk). git index-pack and dulwich's own streaming DeltaChainIterator both reject delta cycles; only this random-access path did not.
3. Track the REF_DELTA base offsets already followed on the chain and raise UnresolvedDeltas when one repeats. Every cycle crosses at least one REF_DELTA edge (OFS_DELTA offsets strictly decrease), so this is enough to break any cycle, and valid chains visit distinct offsets so their result is unchanged.

Verified with a two-object REF_DELTA cycle: before the change get_raw hangs; after it raises UnresolvedDeltas. Added a regression test covering that pack.

[jelmer]

What does C Git do here?