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

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>