dir.c: remove the second declaration of "stk" in prep_exclude()

This "stk" shadows the first declaration at the top. There's currently
no bad effect. But let's avoid it.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

dir.c

@@ -816,9 +816,9 @@ static void prep_exclude(struct dir_struct *dir, const char *base, int baselen)
 	/* Read from the parent directories and push them down. */
 	current = stk ? stk->baselen : -1;
 	while (current < baselen) {
-		struct exclude_stack *stk = xcalloc(1, sizeof(*stk));
 		const char *cp;
 
+		stk = xcalloc(1, sizeof(*stk));
 		if (current < 0) {
 			cp = base;
 			current = 0;