feat: add diff gutter to picker preview

[forivall]

This could probably be improved with some configuration options, if desired.

Screenshot taken from my branch with a few other modifications, notably this hack: forivall@c37b18d4 to select the first change in the git diff picker.

[forivall]

adapted from

helix/helix-term/src/ui/editor.rs

Lines 671 to 706 in 6fffaf6

	let mut gutter = gutter_type.style(editor, doc, view, theme, is_focused);
	let width = gutter_type.width(view, doc);
	// avoid lots of small allocations by reusing a text buffer for each line
	let mut text = String::with_capacity(width);
	let cursors = cursors.clone();
	let gutter_decoration = move |renderer: &mut TextRenderer, pos: LinePos| {
	// TODO handle softwrap in gutters
	let selected = cursors.contains(&pos.doc_line);
	let x = viewport.x + offset;
	let y = pos.visual_line;
	let gutter_style = match (selected, pos.first_visual_line) {
	(false, true) => gutter_style,
	(true, true) => gutter_selected_style,
	(false, false) => gutter_style_virtual,
	(true, false) => gutter_selected_style_virtual,
	};
	if let Some(style) =
	gutter(pos.doc_line, selected, pos.first_visual_line, &mut text)
	{
	renderer.set_stringn(x, y, &text, width, gutter_style.patch(style));
	} else {
	renderer.set_style(
	Rect {
	x,
	y,
	width: width as u16,
	height: 1,
	},
	gutter_style,
	);
	}
	text.clear();
	};
	decoration_manager.add_decoration(gutter_decoration);

, could be fine tuned for the preview use case, or deduplicated and shared.

[senekor]

I think this does need to be configurable, because the normal diff gutter can already be hidden. It would be weird if a normal buffer doesn't have the diff gutter while the picker has.

Maybe it could read the existing editor.gutters list and if it contains "diff", then it also activates it in the picker. That would make things consistent without introducing a new config option.