patch 8.2.4335: no autocommand event triggered before changing directory

Problem:    No autocommand event triggered before changing directory. (Ronnie
            Magatti)
Solution:   Add DirChangedPre. (closes #9721)
diff --git a/src/autocmd.c b/src/autocmd.c
index a10be9b..97b27ef 100644
--- a/src/autocmd.c
+++ b/src/autocmd.c
@@ -119,6 +119,7 @@
     {"CursorMovedI",	EVENT_CURSORMOVEDI},
     {"DiffUpdated",	EVENT_DIFFUPDATED},
     {"DirChanged",	EVENT_DIRCHANGED},
+    {"DirChangedPre",	EVENT_DIRCHANGEDPRE},
     {"EncodingChanged",	EVENT_ENCODINGCHANGED},
     {"ExitPre",		EVENT_EXITPRE},
     {"FileEncoding",	EVENT_ENCODINGCHANGED},