config: fix leak in git_config_get_expiry_in_days()

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>