patch 9.0.0346: :horizontal modifier not fully supported

Problem:    :horizontal modifier not fully supported.
Solution:   Also use :horizontal for completion and user commands.
            (closes #11025)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 681ab2c..9f7875f 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -2455,6 +2455,7 @@
 	    case CMD_final:
 	    case CMD_help:
 	    case CMD_hide:
+	    case CMD_horizontal:
 	    case CMD_ijump:
 	    case CMD_ilist:
 	    case CMD_isearch: