refactor: replace deprecated nvim_buf_get/set_option with vim.bo

[dlyongemallo]

No description provided.

[Copilot]

Pull request overview

Refactors Diffview’s buffer option handling to avoid deprecated nvim_buf_get_option / nvim_buf_set_option calls by using Neovim’s Lua option interface (vim.bo) instead.

Changes:

• Replace buffer option writes in file/panel buffer creation with vim.bo[bufnr][opt] = value.
• Replace buffer modifiable get/set in the renderer with vim.bo[bufid].modifiable.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
lua/diffview/vcs/file.lua	Switches buffer-local option assignment for diff buffers and the NULL buffer to vim.bo.
lua/diffview/ui/panel.lua	Switches panel buffer option assignment to vim.bo.
lua/diffview/renderer.lua	Uses vim.bo to temporarily toggle modifiable during render updates.

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