Apply treesitter highlighting to diff metadata lines (diff --git, index,
---, +++) using the diff language parser. Header info is attached only
to the first hunk of each file to avoid duplicate highlighting.
Based on PR #52 by @phanen with fixes:
- header_lines now only contains diff metadata, not hunk content
- header info attached only to first hunk per file
- removed arbitrary hunk count restriction
Treesitter parses diff hunks in isolation without surrounding code
context, which can cause incorrect highlighting when hunks show partial
blocks (e.g., adding lines inside `return { ... }` without seeing the
`return`). Document this as a known limitation in README and vimdoc.
