run-command API: don't fall back on online_cpus()

run-command API: don't fall back on online_cpus()

When a "jobs = 0" is passed let's BUG() out rather than fall back on
online_cpus(). The default behavior was added when this API was
implemented in c553c72eed6 (run-command: add an asynchronous parallel
child processor, 2015-12-15).

Most of our code in-tree that scales up to "online_cpus()" by default
calls that function by itself. Keeping this default behavior just for
the sake of two callers means that we'd need to maintain this one spot
where we're second-guessing the config passed down into pp_init().

The preceding commit has an overview of the API callers that passed
"jobs = 0". There were only two of them (actually three, but they
resolved to these two config parsing codepaths).

The "fetch.parallel" caller already had a test for the
"fetch.parallel=0" case added in 0353c688189 (fetch: do not run a
redundant fetch from submodule, 2022-05-16), but there was no such
test for "submodule.fetchJobs". Let's add one here.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>