Fix git-for-each-refs broken for tags

Unfortunately, git-for-each-refs is currently unusable for peeking into tag
comments, since it uses freed pointers, so it just prints out all sort of
garbage.

This makes it strdup() contents and body values.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-for-each-ref.c

@@ -478,9 +478,9 @@ static void grab_sub_body_contents(struct atom_value *val, int deref, struct obj
 		if (!strcmp(name, "subject"))
 			v->s = copy_line(subpos);
 		else if (!strcmp(name, "body"))
-			v->s = bodypos;
+			v->s = xstrdup(bodypos);
 		else if (!strcmp(name, "contents"))
-			v->s = subpos;
+			v->s = xstrdup(subpos);
 	}
 }