git_parse_unsigned: reject negative values

git_parse_unsigned: reject negative values

git_parse_unsigned() relies on strtoumax() which unfortunately parses
negative values as large positive integers. Fix this by rejecting any
string that contains '-' as we do in strtoul_ui(). I've chosen to treat
negative numbers as invalid input and set errno to EINVAL rather than
ERANGE one the basis that they are never acceptable if we're looking for
a unsigned integer. This is also consistent with the existing behavior
of rejecting "1–2" with EINVAL.

As we do not have unit tests for this function it is tested indirectly
by checking that negative values of reject for core.bigFileThreshold are
rejected. As this function is also used by OPT_MAGNITUDE() a test is
added to check that rejects negative values too.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Taylor Blau <me@ttaylorr.com>