Add VS Code-style "Group by Staging" mode to the git panel

[PSU3D0]

Summary

Adds a VS Code-style "Group by Staging" mode to the git panel, allowing users to see their changes organized into Staged Changes and Changes sections instead of the current Conflicts / Tracked / Untracked grouping.

This is controlled by a new git_panel.group_by setting:

{
  "git_panel": {
    "group_by": "staging"
  }
}

Behavior

• "status" (default): Preserves the existing grouping — Conflicts, Tracked, Untracked. No behavior change.
• "staging": Groups entries into "Staged Changes" and "Changes" sections.
  • Partially staged files appear in both sections.
  • Clicking a file under Staged Changes opens a diff showing only what's staged (HEAD → Index).
  • Clicking a file under Changes opens a diff showing only unstaged modifications (Index → Worktree).
  • Toggle between modes via right-click context menu on the git panel header.

Screenshots

Implementation

New setting (settings_content, git_panel_settings)

• GitPanelGroupBy enum with Status and Staging variants
• group_by field on GitPanelSettings, default Status

Git panel grouping (git_panel.rs)

• Extended Section enum with Staged / Unstaged variants
• update_visible_entries() classifies files by StageStatus when group_by == Staging
• section_for_entry() walks backwards from a selected entry to find its section header
• open_diff() detects whether the clicked entry is in a Staged or Unstaged section and opens the appropriate diff view
• ToggleGroupByStaging action + context menu entry for toggling

Diff pipeline (branch_diff.rs, git_store.rs, project_diff.rs)

• New DiffBase::Staged and DiffBase::Unstaged variants
• BranchDiff::load_buffers() filters files by staging status for the new modes
• BranchDiff::load_buffer() dispatches to the correct diff loader per mode:
  • Staged → GitStore::open_staged_diff() (new) — creates a read-only buffer with index content, diffs against HEAD
  • Unstaged → GitStore::open_unstaged_diff() (existing) — diffs index against worktree
  • Head / Merge → existing behavior unchanged
• ProjectDiff::deploy_with_base() opens or reuses a ProjectDiff for a given DiffBase variant
• Tab titles: "Staged Changes" / "Unstaged Changes"

Tests

• 5 git panel tests: basic grouping, partially staged files, tree view mode, header toggling, empty section hiding
• 3 project diff tests: staged diff content (HEAD→Index), unstaged diff content (Index→Worktree), staged-only file filtering

Release Notes:

• Added a git_panel.group_by setting. When set to "staging", the git panel groups entries into "Staged Changes" and "Changes" sections (similar to VS Code). Clicking entries in each section shows the appropriate diff — staged changes show HEAD→Index, unstaged changes show Index→Worktree.

[fazulk]

please, we need it

[cole-miller]

Thanks @PSU3D0 for opening a PR--this feature has been added to Zed in #59884 and will be in the next stable release.