config: factor out integer parsing from range checks

config: factor out integer parsing from range checks

When we are parsing integers for config, we use an intmax_t
(or uintmax_t) internally, and then check against the size
of our result type at the end. We can parameterize the
maximum representable value, which will let us re-use the
parsing code for a variety of range checks.

Unfortunately, we cannot combine the signed and unsigned
parsing functions easily, as we have to rely on the signed
and unsigned C types internally.

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