[GSoC][RFC] New approach: improve threading at git-grep

Hi, everyone

I've been working on making the pack access code thread safe, as my GSoC project[1]. I was working directly at sha1-file.c global states targeting, in the far distance, to parallelize git-blame. But my mentors and I decided to take a different path, as a better way to continuous validate the work being done and be able to see small continuous improvements: the idea is to focus on the object store call chains originated at git-grep. Making them thread-safe, we should be able to re-enable threading when greping blobs[2] and go refining git-grep's locks along the way, for a better performance.

I just posted about the overall idea together with some analysis I've made on git-grep, here: https://matheustavares.gitlab.io/posts/week-4-a-different-approach Please, let me know if you have any comments or suggestions.

Thanks, Matheus

[1]: https://matheustavares.gitlab.io/assets/Matheus_Tavares_GSoC_Proposal.pdf [2]: Disabled by 53b8d93 (“grep: disable threading in non-worktree case”, 2011-12-12), because it was causing slowdowns