patch 8.1.0397: no event triggered after updating diffs

Problem:    No event triggered after updating diffs.
Solution:   Add the DiffUpdated event.
diff --git a/src/fileio.c b/src/fileio.c
index d7e05bf..93dbf0c 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -7761,6 +7761,7 @@
     {"CursorHoldI",	EVENT_CURSORHOLDI},
     {"CursorMoved",	EVENT_CURSORMOVED},
     {"CursorMovedI",	EVENT_CURSORMOVEDI},
+    {"DiffUpdated",	EVENT_DIFFUPDATED},
     {"DirChanged",	EVENT_DIRCHANGED},
     {"EncodingChanged",	EVENT_ENCODINGCHANGED},
     {"ExitPre",		EVENT_EXITPRE},