git_ui: Enable navigation for single-hunk diffs

[macayu17]

What

Keep the hunk navigation controls available when a diff contains one hunk, so users can return to it after scrolling away. This applies to solo, staged, unstaged, and project diff views.

Why

The controls were only rendered when hunk_count > 1, even though the existing navigation actions already wrap and recenter a single hunk.

How

Render the previous and next hunk controls whenever hunk_count > 0, reusing the existing navigation actions.

Closes #62469

Testing

• cargo +stable-x86_64-pc-windows-gnu test -p git_ui --lib — 130 passed.
• cargo +stable-x86_64-pc-windows-gnu build -p zed -j 1 — passed.
• rustfmt --edition 2024 --check on the four changed files — passed.
• git diff --check — passed.
• Manually verified with the branch-built Zed on Windows: opened a tracked file with exactly one diff hunk, expanded the context, scrolled below the hunk, and used Go to Previous Hunk. The view returned to the changed line.

Showcase

single-hunk-navigation-clean.mp4

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

Release Notes:

• Fixed hunk navigation being disabled for single-hunk diffs.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[dinocosta]

Hey @macayu17 ! Thank you for taking a stab at this 🙂

Sorry to ask but I'm wondering if you did manage to test these changes yourself, given the following in the Pull Request description, which makes it seem like you were unable to compile the code on your end.

Attempted cargo test -p git_ui solo_diff_view::tests::test_changes_only_multibuffer_has_one_buffer_and_expand_controls; compilation could not complete because MSVC link.exe is unavailable in the local environment.

If possible, can you attach a screen recording showing the updated UI? Thanks!

[macayu17]

Thanks for checking. I rebuilt the branch using the Windows GNU toolchain and ran the full git_ui library suite. All 130 tests passed. I also verified the single-hunk navigation in the branch-built app and attached a screen recording above.