patch 8.2.1635: no digraph for 0x2022 BULLET

Problem:    No digraph for 0x2022 BULLET.
Solution:   Use "oo". (Hans Ginzel, closes #6904)
diff --git a/src/digraph.c b/src/digraph.c
index 7e6128d..fd78a16 100644
--- a/src/digraph.c
+++ b/src/digraph.c
@@ -1173,6 +1173,7 @@
 	{'9', '"', 0x201f},
 	{'/', '-', 0x2020},
 	{'/', '=', 0x2021},
+	{'o', 'o', 0x2022},
 	{'.', '.', 0x2025},
 	{',', '.', 0x2026},
 	{'%', '0', 0x2030},