patch 8.0.0388: filtering lines changes folds

Problem:    filtering lines through "cat", without changing the line count,
            changes manual folds.
Solution:   Change how marks and folds are adjusted. (Matthew Malcomson, from
            neovim #6194.
diff --git a/src/fold.c b/src/fold.c
index d3635a6..cba1375 100644
--- a/src/fold.c
+++ b/src/fold.c
@@ -1576,16 +1576,23 @@
 		{
 		    /* 5. fold is below line1 and contains line2; need to
 		     * correct nested folds too */
-		    foldMarkAdjustRecurse(&fp->fd_nested, line1 - fp->fd_top,
-				  line2 - fp->fd_top, amount,
-				  amount_after + (fp->fd_top - top));
 		    if (amount == MAXLNUM)
 		    {
+			foldMarkAdjustRecurse(&fp->fd_nested,
+				  line1 - fp->fd_top,
+				  line2 - fp->fd_top,
+				  amount,
+				  amount_after + (fp->fd_top - top));
 			fp->fd_len -= line2 - fp->fd_top + 1;
 			fp->fd_top = line1;
 		    }
 		    else
 		    {
+			foldMarkAdjustRecurse(&fp->fd_nested,
+				  line1 - fp->fd_top,
+				  line2 - fp->fd_top,
+				  amount,
+				  amount_after - amount);
 			fp->fd_len += amount_after - amount;
 			fp->fd_top += amount;
 		    }