patch 8.2.1591: using winheight('.') in tests works but is wrong

Problem:    Using winheight('.') in tests works but is wrong.
Solution:   Use winheight(0). (issue #6863)
diff --git a/src/version.c b/src/version.c
index 53a3ed0..60c6294 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1591,
+/**/
     1590,
 /**/
     1589,