Git blobless mode

[9128305]

Hi, is it possible to automatically download blobs when using Jujutsu? Now I need to manually download a blob by commit id (using git cat-file).

[anguslees]

I have a PoC patch that I use on a blobless colocated repo: #6451

I want to emphasise that this PR is incomplete and crashes (with a gitoxide "slotmap too small" error) when it downloads too many blobs on demand. And anecdotally, jj gets noticeably slower when you have many pack files (until git gc), presumably because gitoxide lacks some of git's newer pack indices.