parse: fix off-by-one for minimum signed values

parse: fix off-by-one for minimum signed values

We accept a maximum value in `git_parse_signed()` that restricts the
range of accepted integers. As the intent is to pass `INT*_MAX` values
here, this maximum doesn't only act as the upper bound, but also as the
implicit lower bound of the accepted range.

This lower bound is calculated by negating the maximum. But given that
the maximum value of a signed integer with N bits is `2^(N-1)-1` whereas
the minimum value is `-2^(N-1)` we have an off-by-one error in the lower
bound.

Fix this off-by-one error by using `-max - 1` as lower bound instead.

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