Improve Git performance in large repositories

[aaravrav]

Objective

• Improve Git responsiveness in very large repositories such as nixpkgs.
• Prevent branch switching from waiting for expensive filesystem-driven repository refreshes.
• Prevent loading large commit histories from making the UI choppy or unresponsive.

Solution

• After a branch checkout succeeds, update Zed’s repository state immediately and notify anything observing it. This avoids waiting for a filesystem event and removes an extra Git command that was previously needed to rediscover the active branch.
• When more than 100 paths change at once, refresh Git status from the repository root instead of processing every path individually. This prevents branch switches in large repositories from triggering thousands of incremental status updates.
• Keep at most one commit-history batch queued at a time and give the UI an opportunity to run between batches. This prevents a large history load from overwhelming the foreground thread.
• Load commit history only after the History tab is opened instead of preloading it while the user is viewing Changes.
• As history batches arrive, request and append only the newly available commits instead of rebuilding the entire list.
• When drawing the history list, process only the visible rows instead of copying every loaded commit.

Testing

• Tested branch switching and large commit-history loading manually with the nixpkgs repository on my linux machine
• Verified that branch switching completes promptly and the History tab remains responsive while loading many commits.
• Ran the relevant branch-switching, commit-history, and path-coalescing tests.
• I didn't test other platforms. Reviewers can test by opening a large repository like nixpkgs, switching branches, and loading a long commit history.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

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Release Notes:

• Improved Git panel responsiveness and branch switching in large repositories.

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[aaravrav]

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[aaravrav]

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