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/proto/digraph.pro b/src/proto/digraph.pro
index 4c12f51..37dc2b2 100644
--- a/src/proto/digraph.pro
+++ b/src/proto/digraph.pro
@@ -4,7 +4,7 @@
 int get_digraph(int cmdline);
 int getdigraph(int char1, int char2, int meta_char);
 void putdigraph(char_u *str);
-void listdigraphs(void);
+void listdigraphs(int use_headers);
 char_u *keymap_init(void);
 void ex_loadkeymap(exarg_T *eap);
 void keymap_clear(garray_T *kmap);