config: fix leak in git_config_get_expiry_in_days()

We use git_config_get_string() to retrieve the expiry value in a newly
allocated string. But after parsing it, we never free it, leaking the
memory.

We could fix this with a free() obviously, but there's an even better
solution: we can use the non-allocating "tmp" variant of the function;
we only need it to be valid for the lifetime of our parse function.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -2300,11 +2300,11 @@ int git_config_get_expiry(const char *key, const char **output)
 
 int git_config_get_expiry_in_days(const char *key, timestamp_t *expiry, timestamp_t now)
 {
-	char *expiry_string;
+	const char *expiry_string;
 	intmax_t days;
 	timestamp_t when;
 
-	if (git_config_get_string(key, &expiry_string))
+	if (git_config_get_string_tmp(key, &expiry_string))
 		return 1; /* no such thing */
 
 	if (git_parse_signed(expiry_string, &days, maximum_signed_value_of_type(int))) {