My second take on uncluttering the git graph after #53692, this time by
simply replacing `git log --all` with `git log --ignore-missing
--branches --remotes --tags HEAD`. Thanks @JonGretar for educating me on
this 🙂

This pretty much mirrors which commits VSCode's built-in git graph shows
when selecting "All history item references" (though VSCode has an extra
`git for-each-ref` step):
https://github.com/microsoft/vscode/blob/341ef7db2e1d95f96c9f63c40a518d25bb455188/extensions/git/src/git.ts#L1280-L1344

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:

- Git Graph: Now excludes commits that aren't reachable from branches or
tags.