Support iteration through file deltas.

[turbocool3r]

Summary 💡

I'm suggesting exposing delta chains from packs in a repository to accelerate incremental parsing of code for analysis. This should provide some API that returns either a full object or a structure wrapping a delta when odb is queried for an oid. Ideally this should support ref deltas.

Motivation 🔦

I'm trying to write a tool that uses an incremental parser to statically analyze code in large Git repos fast. Since large repos are usually compressed into packs and downloaded as packs (IIRC) identifying chains of deltas that can be fed into a parser would likely provide a considerable speedup.