patch 8.1.0586: :digraph output is not easy to read

Problem:    :digraph output is not easy to read.
Solution:   Add highlighting for :digraphs. (Marcin Szamotulski, closes #3572)
            Also add section headers for :digraphs!.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 99a06fc..c738d07 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -12413,7 +12413,7 @@
     if (*eap->arg != NUL)
 	putdigraph(eap->arg);
     else
-	listdigraphs();
+	listdigraphs(eap->forceit);
 #else
     EMSG(_("E196: No digraphs in this version"));
 #endif