Adds gdiff_file() which can diff any file path (not just current buffer)
with support for staged vs unstaged changes:
- staged=true diffs index against HEAD
- staged=false diffs working tree against index
- Falls back to HEAD if file not in index (for untracked comparison)
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.
