cc @SomeoneToIgnore

## Summary

Follow-up to https://github.com/zed-industries/zed/discussions/55352,
where the conclusion was to split `editor.rs` incrementally by topic
instead of all at once.

This mechanically extracts two editor topics into focused sibling
modules:

- `crates/editor/src/input.rs`
- `crates/editor/src/git.rs`

The git extraction is intentionally partial for now. I left a lot of
related parts because otherwise the diff was super huge (over 9K lines)
in the Github, so we can move those parts later in the follow-up PRs
Self-Review Checklist:

- [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