patch 9.0.0115: when 'cmdheight' is zero pressing ':' may scroll a window

Problem:    When 'cmdheight' is zero pressing ':' may scroll a window.
Solution:   Add the made_cmdheight_nonzero flag and set 'scrolloff' to zero.
diff --git a/src/window.c b/src/window.c
index 15a4d50..e418f11 100644
--- a/src/window.c
+++ b/src/window.c
@@ -6365,7 +6365,8 @@
 
     // There is no point in adjusting the scroll position when exiting.  Some
     // values might be invalid.
-    if (!exiting)
+    // Skip scroll_to_fraction() when 'cmdheight' was set to one from zero.
+    if (!exiting && !made_cmdheight_nonzero)
 	scroll_to_fraction(wp, prev_height);
 }