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/cmdexpand.c b/src/cmdexpand.c
index 5cdb1be..bfa3053 100644
--- a/src/cmdexpand.c
+++ b/src/cmdexpand.c
@@ -1776,6 +1776,7 @@
 	case CMD_folddoclosed:
 	case CMD_folddoopen:
 	case CMD_hide:
+	case CMD_horizontal:
 	case CMD_keepalt:
 	case CMD_keepjumps:
 	case CMD_keepmarks: