Keep the horizontal scroll position when a diff is rendered again

Two places that position the main view after a re-render of the same
diff call SetOrigin(0, y), although they only mean to move the view
vertically. One is the restore after a change of the context size, of
whitespace handling or of the diff renderer. The other is the clamp for
content that ends above the scroll position, for example after staging a
large part of the diff. Use SetOriginY in both, so that they keep the
horizontal scroll position.

Nothing scrolls the main view horizontally yet, so this changes nothing
on its own. It prepares for scrolling the focused main view sideways
with H and L.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

pkg/gui/controllers/helpers/diff_line_restore.go

@@ -81,7 +81,7 @@ func (self *DiffLineHelper) PreserveDiffPositionOnRerender(view *gocui.View) {
 			// Put the line back on the screen row it was on, clamped into the view for
 			// the fallback lines, which can come from off screen.
 			row := lo.Clamp(anchor.row, 0, max(0, view.InnerHeight()-1))
-			view.SetOrigin(0, max(0, viewLine-row))
+			view.SetOriginY(max(0, viewLine-row))
 			if showSelection {
 				// Put the far end back before the cursor, so that the selection covers
 				// the same lines again; a selection whose far end didn't survive the