fix: guard against external autocmd failures on buffer set

[dlyongemallo]

Workaround for an error message originating from nvim-lualine/lualine.nvim#699.

[Copilot]

Pull request overview

This PR adds a safer wrapper around nvim_win_set_buf() to recover from failures caused by external autocommands, and integrates it into Diffview buffer/panel window setup to avoid hard failures during view open.

Changes:

• Introduce utils.set_win_buf() to retry nvim_win_set_buf() once with window/buffer events ignored.
• Replace direct api.nvim_win_set_buf() calls with utils.set_win_buf() in key buffer-setting paths.
• Add functional tests for utils.set_win_buf() and add a one-time user warning + logging when recovery happens while opening a Diffview buffer.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lua/diffview/utils.lua	Adds set_win_buf() helper with a retry path using no_win_event_call().
lua/diffview/scene/window.lua	Uses set_win_buf() for opening diff buffers; warns/logs when recovery occurs and falls back on failure.
lua/diffview/vcs/file.lua	Uses set_win_buf() when loading the null buffer into a window.
lua/diffview/ui/panel.lua	Uses set_win_buf() when opening the panel split and assigning its buffer.
lua/diffview/tests/functional/utils_spec.lua	Adds functional tests for set_win_buf() behavior across success/retry/failure cases.

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