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_cmds.h b/src/ex_cmds.h
index 335871a..49e7fc7 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -456,7 +456,7 @@
 			TRLBAR,
 			ADDR_LINES),
 EX(CMD_digraphs,	"digraphs",	ex_digraphs,
-			EXTRA|TRLBAR|CMDWIN,
+			BANG|EXTRA|TRLBAR|CMDWIN,
 			ADDR_LINES),
 EX(CMD_djump,		"djump",	ex_findpat,
 			BANG|RANGE|DFLALL|WHOLEFOLD|EXTRA,