tests: introduce --stress-jobs=<N>
The --stress option currently accepts an argument, but it is confusing
to at least this user that the argument does not define the maximal
number of stress iterations, but instead the number of jobs to run in
parallel per stress iteration.
Let's introduce a separate option for that, whose name makes it more
obvious what it is about, and let --stress=<N> error out with a helpful
suggestion about the two options tha could possibly have been meant.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>