config: set errno in numeric git_parse_* functions

config: set errno in numeric git_parse_* functions

When we are parsing an integer or unsigned long, we use
the strto*max functions, which properly set errno to ERANGE
if we get a large value. However, we also do further range
checks after applying our multiplication factor, but do not
set ERANGE. This means that a caller cannot tell if an error
was caused by ERANGE or if the input was simply not a valid
number.

This patch teaches git_parse_signed and git_parse_unsigned to set
ERANGE for range errors, and EINVAL for other errors, so that the
caller can reliably tell these cases apart.

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