config: use git_parse_int() in git_config_get_expiry_in_days()

config: use git_parse_int() in git_config_get_expiry_in_days()

git_config_get_expiry_in_days() calls git_parse_signed() with the
maximum value of int, which is equivalent to calling git_parse_int().
Do that instead, as its shorter and clearer.

This requires demoting "days" to int to match.  Promote "scale" to
intmax_t in turn to arrive at the same result when multiplying them.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>