patch 8.2.0909: cannot go back to the previous local directory

Problem:    Cannot go back to the previous local directory.
Solution:   Add "tcd -" and "lcd -". (Yegappan Lakshmanan, closes #4362)
diff --git a/src/filepath.c b/src/filepath.c
index ad7b8b0..8bc941d 100644
--- a/src/filepath.c
+++ b/src/filepath.c
@@ -726,6 +726,7 @@
 
     if (argvars[0].v_type != VAR_STRING)
 	// Returning an empty string means it failed.
+	// No error message, for historic reasons.
 	return;
 
     // Return the current directory