patch 8.1.2315: not always using the right window when jumping to an error

Problem:    Not always using the right window when jumping to an error.
Solution:   Add the "uselast" flag in 'switchbuf'. (closes #1652)
diff --git a/src/quickfix.c b/src/quickfix.c
index a5d307e..aa4e765 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -3013,8 +3013,11 @@
 	if (IS_QF_WINDOW(win))
 	{
 	    // Didn't find it, go to the window before the quickfix
-	    // window.
-	    if (altwin != NULL)
+	    // window, unless 'switchbuf' contains 'uselast': in this case we
+	    // try to jump to the previously used window first.
+	    if ((swb_flags & SWB_USELAST) && win_valid(prevwin))
+		win = prevwin;
+	    else if (altwin != NULL)
 		win = altwin;
 	    else if (curwin->w_prev != NULL)
 		win = curwin->w_prev;