patch 8.2.0077: settagstack() cannot truncate at current index

Problem:    settagstack() cannot truncate at current index.
Solution:   Add the "t" action. (Yegappan Lakshmanan, closes #5417)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 489ab42..af43fb9 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -6776,7 +6776,8 @@
 	actstr = tv_get_string_chk(&argvars[2]);
 	if (actstr == NULL)
 	    return;
-	if ((*actstr == 'r' || *actstr == 'a') && actstr[1] == NUL)
+	if ((*actstr == 'r' || *actstr == 'a' || *actstr == 't')
+		&& actstr[1] == NUL)
 	    action = *actstr;
 	else
 	{