config: avoid calling `labs()` on too-large data type

The `labs()` function operates, as the initial `l` suggests, on `long`
parameters. However, in `config.c` we tried to use it on values of type
`intmax_t`.

This problem was found by GCC v9.x.

To fix it, let's just "unroll" the function (i.e. negate the value if it
is negative).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -869,9 +869,9 @@ static int git_parse_signed(const char *value, intmax_t *ret, intmax_t max)
 			errno = EINVAL;
 			return 0;
 		}
-		uval = labs(val);
+		uval = val < 0 ? -val : val;
 		uval *= factor;
-		if (uval > max || labs(val) > uval) {
+		if (uval > max || (val < 0 ? -val : val) > uval) {
 			errno = ERANGE;
 			return 0;
 		}