submodule update: expose parallelism to the user

submodule update: expose parallelism to the user

Expose possible parallelism either via the "--jobs" CLI parameter or
the "submodule.fetchJobs" setting.

By having the variable initialized to -1, we make sure 0 can be passed
into the parallel processing machine, which will then pick as many parallel
workers as there are CPUs.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>