feat(view): add focus_diff option, fix selected_row focus

[dlyongemallo]

No description provided.

[Copilot]

Pull request overview

Adds a new per-view configuration option to control whether Diffview focuses the diff window on initial open, and adjusts initial selected_row positioning to focus the main diff window.

Changes:

• Introduce view.<section>.focus_diff (default/merge_tool/file_history) with default false.
• Route initial focus in DiffView:update_files() and FileHistoryView:post_open() based on focus_diff.
• Add functional specs covering config defaults/overrides, focus routing, and selected_row focusing behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
lua/diffview/config.lua	Adds focus_diff = false defaults to each view section.
lua/diffview/scene/views/diff/diff_view.lua	Uses focus_diff to decide whether to focus diff window on first open; focuses main win when applying selected_row.
lua/diffview/scene/views/file_history/file_history_view.lua	Passes view.file_history.focus_diff into set_file() during initial open.
lua/diffview/tests/functional/focus_diff_spec.lua	Adds new functional tests for config + focus behavior (with some test maintainability concerns).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.