Section headers (Staged/Unstaged) now show all diffs in that section,
matching fugitive's behavior. Untracked files show as all-added diffs.
Deleted files show as all-removed diffs.
Also handles edge cases:
- Empty new/old content for deleted/new files
- Section header detection returns is_header flag
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.
