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>