config: properly range-check integer values

When we look at a config value as an integer using the
git_config_int function, we carefully range-check the value
we get and complain if it is out of our range. But the range
we compare to is that of a "long", which we then cast to an
"int" in the function's return value. This means that on
systems where "int" and "long" have different sizes (e.g.,
LP64 systems), we may pass the range check, but then return
nonsense by truncating the value as we cast it to an int.

We can solve this by converting git_parse_long into
git_parse_int, and range-checking the "int" range. Nobody
actually cared that we used a "long" internally, since the
result was truncated anyway, and there were no other callers
of git_parse_long.

Of the existing callers, all of them expect small-ish values
that are fine for an "int", and should not be impacted
except when nonsense is fed to them. The one arguable
exception is http.lowSpeedLimit, which is given in bytes per
second, and is fed to curl as a long. However, nobody is
likely to consider 2GB/sec as "low speed", and even if they
did, the new behavior is much better (it barfs and complains
rather than considering the value negative and silently
ignoring it).
teach git-config to output large integers

Internally we use "unsigned long" to represent large config
values like packed window memory sizes, packfile size
limits, etc.  On 32-bit systems, this limits these config
options to 4G (and we detect and complain about overflow).
On 64-bit systems, they get the full 64-bit range.

However, there is no way for script callers of git-config to
see the numeric values (they cannot treat them as pure
strings, since they would want git-config to process "3g"
into "3221225472").

This patch teaches git-config a "--ulong" type that can
output these unsigned long values, allowing scripts to
examine the values as the internal code would.