Compares current buffer against any git revision (default HEAD), opens result
with full diffs.nvim syntax highlighting. Follows fugitive convention:
:Gdiff/:Gvdiff open vertical split, :Ghdiff opens horizontal split.
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
