patch 9.0.1309: scrolling two lines with even line count and 'scrolloff' set

Problem:    Scrolling two lines with even line count and 'scrolloff' set.
Solution:   Adjust how the topline is computed. (closes #10545)
diff --git a/src/normal.c b/src/normal.c
index 9a30067..f5a85ac 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -2712,7 +2712,7 @@
     case '.':	beginline(BL_WHITE | BL_FIX);
 		// FALLTHROUGH
 
-    case 'z':	scroll_cursor_halfway(TRUE);
+    case 'z':	scroll_cursor_halfway(TRUE, FALSE);
 		redraw_later(UPD_VALID);
 		set_fraction(curwin);
 		break;