fix(file): suppress FileType during synthetic BufReadPost

[dlyongemallo]

Fixes #113.

[Copilot]

Pull request overview

This PR fixes a folding regression triggered by synthetic BufReadPost emission in File:attach_buffer() by preventing Neovim’s filetypedetect augroup from indirectly re-firing FileType, which can cause user FileType autocmds (e.g., treesitter folding setup) to override Diffview’s window-local options (ref #113).

Changes:

• Temporarily appends FileType to eventignore while executing the synthetic BufReadPost autocmd for diff buffers.
• Refactors the BufReadPost emission guard into a local should_emit_buf_read boolean for readability.
• Adds/updates functional tests to cover: skipping emission when diffview_disable_ts is set, emitting only once across repeated attaches, suppressing FileType, and restoring eventignore.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lua/diffview/vcs/file.lua	Suppresses FileType during the synthetic BufReadPost to prevent folding/window-option clobbering.
lua/diffview/tests/functional/keymap_save_spec.lua	Adds regression and safety tests for BufReadPost emission behavior and eventignore restoration.

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