fix(highlight): avoid leaking diff syntax into active buffers

[carlos-algms]

When Diffview initializes and diffAdded is undefined, it sources Neovim's syntax/diff.vim in the current buffer.

In Tree-sitter-only setups, b:current_syntax can be unset, so this installs buffer-local ^+ and ^- syntax matches.

Unchanged files may then display addition and deletion colors based only on their line prefixes.

• Replace syntax-file sourcing with default highlight links
• Preserve user-defined diff highlight groups
• Add regression coverage for buffer contamination

[Copilot]

Pull request overview

This PR prevents Diffview’s highlight initialization from contaminating non-diff buffers by avoiding runtime! syntax/diff.vim (which can install buffer-local ^+ / ^- syntax matches when b:current_syntax is unset). Instead, it provides safe default highlight links for diffAdded/diffRemoved/diffChanged, while keeping existing user-defined groups intact.

Changes:

• Replace diff-syntax sourcing with default highlight links for diffAdded, diffRemoved, and diffChanged.
• Keep user-defined diff highlight groups unchanged via default = true.
• Add functional regression tests to ensure buffers aren’t polluted by diff syntax setup.

Reviewed changes

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

File	Description
lua/diffview/hl.lua	Replaces syntax/diff.vim loading with safe default highlight links for diff groups.
lua/diffview/tests/functional/highlight_spec.lua	Adds regression coverage for buffer contamination and preservation of existing diff highlight groups.

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[dlyongemallo]

Thanks @carlos-algms for the fix!