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/globals.h b/src/globals.h
index 786c850..86472ac 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1728,3 +1728,6 @@
 // While executing a regexp and set to OPTION_MAGIC_ON or OPTION_MAGIC_OFF this
 // overrules p_magic.  Otherwise set to OPTION_MAGIC_NOT_SET.
 EXTERN optmagic_T magic_overruled INIT(= OPTION_MAGIC_NOT_SET);
+
+// Set when 'cmdheight' is changed from non-zero to one temporarily.
+EXTERN int made_cmdheight_nonzero INIT(= FALSE);