git_graph: Implement basic search functionality (#51886)

## Context

This uses `git log` to get a basic search working in the git graph. This
is one of the last blockers until a full release, the others being
improvements to the graph canvas UI.

## Self-Review Checklist

<!-- Check before requesting review: -->
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

---------

Co-authored-by: Remco Smits <djsmits12@gmail.com>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
852b4fc5f4Anthony Eid committed on 3/27/2026, 3:10:01 PM· committed by GitHubparentcb97ac4
9 files changedLine totals unavailable