scrolling: let Scroll-Up/Down keep the cursor in the same text position

Instead of keeping the cursor in the same spot on the screen,
let the cursor move with the text (whenever possible).

This makes these functions behave the same as in Vim and Emacs.

src/move.c

@@ -493,17 +493,18 @@ void do_up(bool scroll_only)
 						openfile->firstcolumn == 0)
 		return;
 
+	if (scroll_only)
+		edit_scroll(BACKWARD);
+
 	get_edge_and_target(&leftedge, &target_column);
 
 	/* If we can't move up one line or chunk, we're at top of file. */
-	if (go_back_chunks(1, &openfile->current, &leftedge) > 0)
+	if ((!scroll_only || openfile->current_y == editwinrows - 1) &&
+				go_back_chunks(1, &openfile->current, &leftedge) > 0)
 		return;
 
 	set_proper_index_and_pww(&leftedge, target_column, FALSE);
 
-	if (scroll_only)
-		edit_scroll(BACKWARD);
-
 	edit_redraw(was_current, FLOWING);
 
 	/* <Up> should not change placewewant, so restore it. */
@@ -517,17 +518,19 @@ void do_down(bool scroll_only)
 	filestruct *was_current = openfile->current;
 	size_t leftedge, target_column;
 
+	if (scroll_only && (openfile->current_y > 0 ||
+						openfile->current != openfile->filebot))
+		edit_scroll(FORWARD);
+
 	get_edge_and_target(&leftedge, &target_column);
 
 	/* If we can't move down one line or chunk, we're at bottom of file. */
-	if (go_forward_chunks(1, &openfile->current, &leftedge) > 0)
+	if ((!scroll_only || openfile->current_y == 0) &&
+				go_forward_chunks(1, &openfile->current, &leftedge) > 0)
 		return;
 
 	set_proper_index_and_pww(&leftedge, target_column, TRUE);
 
-	if (scroll_only)
-		edit_scroll(FORWARD);
-
 	edit_redraw(was_current, FLOWING);
 
 	/* <Down> should not change placewewant, so restore it. */