config: fix constness of out parameter for `git_config_get_expiry()`

config: fix constness of out parameter for `git_config_get_expiry()`

The type of the out parameter of `git_config_get_expiry()` is a pointer
to a constant string, which creates the impression that ownership of the
returned data wasn't transferred to the caller. This isn't true though
and thus quite misleading.

Adapt the parameter to be of type `char **` and adjust callers
accordingly. While at it, refactor `get_shared_index_expire_date()` to
drop the static `shared_index_expire` variable. It is only used in that
function, and furthermore we would only hit the code where we parse the
expiry date a single time because we already use a static `prepared`
variable to track whether we did parse it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>