git_ui: Open focused single-file diff from commit diff multibuffer

[gaojunran]

Summary

When clicking the Open File button or the context menu Open File Diff inside a CommitView's diff multibuffer, the editor currently opens a plain read-only file view with no diff styling. This is confusing — users expect to see a diff, not a bare file.

This PR extends SoloDiffView to support commit diffs, so both entry points open a focused single-file diff view instead.

• Discussion #62068
• Inspired by and is a follow up of PR #56152 which added SoloDiffView for Git Panel working-tree diffs

Showcase

This PR (has diff style):

2026-08-02.19.01.24.mov

Upstream main (no diff style):

2026-08-02.19.07.44.mov

Changes

solo_diff_view.rs

• Added DiffSource enum (Uncommitted | Commit { sha }) to distinguish working-tree diffs from commit diffs
• Added open_or_focus_commit() — loads the commit diff, filters to a single file, reuses build_buffer/build_buffer_diff from commit_view.rs, and creates a SoloDiffView with DiffSource::Commit
• new() accepts a diff_source parameter; commit diffs use RestoreOnlyDiffHunkDelegate (matching CommitView behavior)
• matches() / matches_commit() — deduplication correctly distinguishes uncommitted and commit diff views
• button_states() early-returns with all staging/restore buttons disabled for commit diffs
• SoloDiffGitToolbar — commit diffs only show Prev/Next Hunk + diff stat; staging/commit buttons hidden via .when(!is_commit_diff, ...)
• SoloDiffStyleToolbar — commit diffs skip the status icon
• breadcrumbs — commit diffs skip status_for_path (no misleading working-tree status)
• Tab content shows "{short_sha} — {file_name}"; tooltip shows full SHA + path

commit_view.rs

• GitBlob, build_buffer, build_buffer_diff changed to pub(crate) for reuse
• RHS editor sets delegate_open_excerpts(true) — intercepts the "Open File" button click, emits OpenExcerptsRequested
• handle_editor_event — on OpenExcerptsRequested, extracts the buffer's file path and opens SoloDiffView::open_or_focus_commit
• clone_on_split — also sets delegate_open_excerpts(true) and subscribes to the event (covers split mode RHS)
• Context menu entry relabeled to "Open File Diff", calls open_or_focus_commit instead of open_path_preview
• OpenFileAtHead action handler also calls open_or_focus_commit
• Extracted open_file_diff helper to eliminate duplicated call pattern
• Removed open_file_at_head (no longer called)

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:

• Added the ability to open a focused single-file diff view from the commit diff multibuffer, showing diff styling instead of a plain read-only file

[MrSubidubi]

We currently enforce a 3 PR limit per contributor in our
repository contribution guidelines and you are currently way above that limit - since this is a draft, I'll for now close this first. Please wait for feedback and merge first, thank you.

Not saying this cannot be opened again, yet please consider this only if you are within limits again. Thank you.