patch 8.0.1459: cannot handle change of directory

Problem:    Cannot handle change of directory.
Solution:   Add the DirChanged autocommand event. (Andy Massimino,
            closes #888)  Avoid changing directory for 'autochdir' too often.
diff --git a/src/fileio.c b/src/fileio.c
index fb5af64..ab216ab 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -7798,6 +7798,7 @@
     {"CursorHoldI",	EVENT_CURSORHOLDI},
     {"CursorMoved",	EVENT_CURSORMOVED},
     {"CursorMovedI",	EVENT_CURSORMOVEDI},
+    {"DirChanged",	EVENT_DIRCHANGED},
     {"EncodingChanged",	EVENT_ENCODINGCHANGED},
     {"FileEncoding",	EVENT_ENCODINGCHANGED},
     {"FileAppendPost",	EVENT_FILEAPPENDPOST},
@@ -9588,7 +9589,7 @@
     {
 	sfname = vim_strsave(fname);
 	/* Don't try expanding FileType, Syntax, FuncUndefined, WindowID,
-	 * ColorScheme or QuickFixCmd* */
+	 * ColorScheme, QuickFixCmd* or DirChanged */
 	if (event == EVENT_FILETYPE
 		|| event == EVENT_SYNTAX
 		|| event == EVENT_FUNCUNDEFINED
@@ -9597,7 +9598,8 @@
 		|| event == EVENT_QUICKFIXCMDPRE
 		|| event == EVENT_COLORSCHEME
 		|| event == EVENT_OPTIONSET
-		|| event == EVENT_QUICKFIXCMDPOST)
+		|| event == EVENT_QUICKFIXCMDPOST
+		|| event == EVENT_DIRCHANGED)
 	    fname = vim_strsave(fname);
 	else
 	    fname = FullName_save(fname, FALSE);