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/usercmd.c b/src/usercmd.c
index 7113511..6b3eadc 100644
--- a/src/usercmd.c
+++ b/src/usercmd.c
@@ -1443,6 +1443,9 @@
     // :vertical
     if (cmod->cmod_split & WSP_VERT)
 	result += add_cmd_modifier(buf, "vertical", multi_mods);
+    // :horizontal
+    if (cmod->cmod_split & WSP_HOR)
+	result += add_cmd_modifier(buf, "horizontal", multi_mods);
     return result;
 }