Fix missing syntax highlighting in proposed diff view

[blink-so]

What & Why

When Claude Code shows a Proposed changes diff, the scratch buffer on the right didn’t inherit the original file’s filetype, so Neovim rendered it without syntax highlighting (issue #20).

Key changes in this PR

1. _detect_filetype helper

   • Tries Neovim’s built-in vim.filetype.match (≥0.10) or falls back to the extension map.
   • Can also read the type from an existing buffer.

2. Filetype propagation

   • After opening the temp/scratch buffer, the detected filetype is applied with vim.api.nvim_set_option_value.

3. Tests

   • Added unit spec to assert the filetype is set (using spy.on) so regressions are caught.

4. Makefile improvements

   • Defines a global NIX_PREFIX that expands to nix develop .#ci -c only when outside a Nix shell, preventing nested shells in CI.

5. Misc

   • Removed a stray blank line in diff.lua docblock.
   • Consolidated commits into a single, clean history.

Verification

nix fmt                                # no changes
nix develop .#ci -c make check         # luacheck 0 warnings / 0 errors
nix develop .#ci -c make test          # 258 tests, 0 failures

---

Fixes #20.

Co-authored-by: ThomasK33 [email protected]

[Copilot]

Pull Request Overview

This PR ensures that proposed diff buffers inherit the original file’s syntax highlighting by detecting and propagating the filetype, and also refines CI shell invocation.

• Introduces a _detect_filetype helper for path‐ and buffer‐based filetype resolution.
• Applies detected filetypes in both native diff and split diff views.
• Adds a unit test for native-diff filetype propagation and updates the Makefile to prevent nested Nix shells in CI.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
tests/unit/diff_spec.lua	Added a test to verify filetype propagation in _open_native_diff.
lua/claudecode/diff.lua	Implemented _detect_filetype and applied filetype in diff views.
Makefile	Defined NIX_PREFIX to skip nix develop when already in a shell.

Comments suppressed due to low confidence (2)

lua/claudecode/diff.lua:721

• Add a unit test for _create_diff_view_from_window to verify that the filetype is correctly propagated in split diff views, matching the native-diff test pattern.

-- Ensure new buffer inherits filetype from original for syntax highlighting (#20)

lua/claudecode/diff.lua:722

• Ensure that original_buffer is defined in this scope before passing it to _detect_filetype. If original_buffer is undefined, this will cause a runtime error.

local original_ft = _detect_filetype(old_file_path, original_buffer)

[Copilot]

[nitpick] Consider extracting the simple_map table to a module-level constant so it isn’t recreated on every call, improving performance and clarity.

Suggested change

	local simple_map = {
	lua = "lua",
	ts = "typescript",
	js = "javascript",
	jsx = "javascriptreact",
	tsx = "typescriptreact",
	py = "python",
	go = "go",
	rs = "rust",
	c = "c",
	h = "c",
	cpp = "cpp",
	hpp = "cpp",
	md = "markdown",
	sh = "sh",
	zsh = "zsh",
	bash = "bash",
	json = "json",
	yaml = "yaml",
	yml = "yaml",
	toml = "toml",
	}

[Copilot]

[nitpick] After spying on nvim_set_option_value, consider restoring or removing the spy in an after_each block to avoid side effects on other tests.