feat: treesitter highlighting for diff headers

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

doc/diffs.nvim.txt

@@ -12,6 +12,7 @@ built-in diff mode.
 
 Features: ~
 - Syntax highlighting in |:Git| summary diffs and commit detail views
+- Diff header highlighting (`diff --git`, `index`, `---`, `+++`)
 - Syntax highlighting in |:Gdiffsplit| / |:Gvdiffsplit| side-by-side diffs
 - Background-only diff colors for any `&diff` buffer (vimdiff, diffthis, etc.)
 - Vim syntax fallback for languages without a treesitter parser
@@ -296,5 +297,13 @@ Checks performed:
 - Neovim version >= 0.9.0
 - vim-fugitive is installed (optional)
 
+==============================================================================
+ACKNOWLEDGEMENTS                                     *diffs-acknowledgements*
+
+- vim-fugitive (https://github.com/tpope/vim-fugitive)
+- codediff.nvim (https://github.com/esmuellert/codediff.nvim)
+- diffview.nvim (https://github.com/sindrets/diffview.nvim)
+- @phanen (https://github.com/phanen) - diff header highlighting
+
 ==============================================================================
  vim:tw=78:ts=8:ft=help:norl: