git_panel: Show staged and unstaged diff stats

[chirivelli]

Objective

• Show accurate diff stats for each staged and unstaged projection of a partially staged file in the Git panel.
• This was originally considered for git_panel: Add group by staging view option #59884, but was scoped out of that already-large PR and is being submitted separately as discussed there.

Solution

• Collect HEAD-to-index and index-to-worktree diff stats alongside the existing combined HEAD-to-worktree stats.
• Carry the staged and unstaged stats through repository status snapshots and remote status serialization.
• Use the stat matching the projected Git panel section while preserving the combined stat for the other grouping modes.
• Update the fake Git repository and add regression coverage with deliberately different staged and unstaged counts.

Testing

• cargo check -p git_ui
• cargo check -p collab
• cargo test -p git_ui test_group_by_staging_section_membership_and_order --lib
• cargo test -p project --lib --no-run
• cargo fmt --all -- --check
• git diff --check

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:

• Fixed diff stats for partially staged files in the Git panel

[chirivelli]

rebased on top of main!
after #60976 got merged

[ChristopherBiscardi]

thanks! Don't forget to include images when making visual change PRs.

before (note whatever.py)

after