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/version.c b/src/version.c
index 2c8b3ac..9e3f720 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    586,
+/**/
     585,
 /**/
     584,