draft: fix: handle varying compression ratios in pack file decoding

[ketsapiwiq]

Hi! I needed to add that fix to be able to clone my large repo with a lot of history in GitJournal.
However, as I don't know Dart or git that extensively, it's an LLM/agent/slop PR (GLM 5) (except this text is written by me).
It fixed my problem but code is probably not in a state you want, however maybe it will be an inspiration so I'm opening this PR as draft. Feel free to close it of course.

Sorry I can't give the repo that I'm now able to clone because, well, it's personal notes :p

Thanks a lot for the beautiful software.

The previous implementation assumed that reading objSize compressed bytes would always yield objSize decompressed bytes. This fails in large repos where compression ratios vary significantly.

Changed to iteratively read chunks until enough decompressed data is available, with proper error handling instead of assertion failure.

Fixes assertion error:
'outputSink.builder.length >= objSize': is not true

[vHanda]

Thanks for this. I'll test it out against my repos, and read a bit more and then probably merge it.

The long term plan is to to migrate to gitoxide and archive this project.