Commit diff multibuffer: "Open File" button opens plain read-only file instead of diff view

[gaojunran]

Problem

When viewing a commit diff (e.g., from Git Panel > History > click a commit), the diff is displayed as a multibuffer showing all changed files. Each file section has a file header with an "Open File" button on the right side, and right-clicking the header shows an "Open File in Project" context menu entry.

Clicking either of these opens the file at HEAD as a plain, read-only editor view — no diff highlighting, no hunk staging controls, no way to see what changed in that commit for that file. This is confusing because the user is in a diff context and expects to see diff styling.

Expected behavior

Like PR #56152 (git_ui: Open file diffs from git panel), which made the Git Panel double-click open a SoloDiffView with full diff styling, the "Open File" button and "Open File in Project" context menu in the commit diff multibuffer should also open a diff view for that specific file — showing the changes introduced by that commit, with diff highlighting and hunk-level controls.

Current code path

• "Open File" button (crates/editor/src/element/header.rs ~line 928): calls editor.open_excerpts_common() → open_buffers_in_workspace() → workspace.open_project_item::<Editor>(). This opens a plain Editor. For non-project files (git blobs at HEAD), set_read_only(true) is called (~line 10212). No diff styling.

• "Open File in Project" context menu (crates/git_ui/src/commit_view.rs ~line 144, CommitDiffAddon::extend_buffer_header_context_menu): calls open_file_at_head() → workspace.open_path_preview(). Also opens a plain file view at HEAD.

• For comparison, the Git Panel (working tree changes) already has git::OpenFileDiff action → SoloDiffView::open_or_focus() (crates/git_ui/src/git_ui.rs ~line 335), which opens a proper diff view. But this is only wired up for files with working-tree status, not for commit diff buffers.

Suggested approach

Extend SoloDiffView (or create a similar view) to support opening a single-file diff for a specific commit, and wire it up as the action behind the "Open File" button and context menu in the commit diff multibuffer.