fetch: choose a sensible default with --jobs=0 again
prior to
51243f9 (run-command API: don't fall back on online_cpus(),
2022-10-12) `git fetch --multiple --jobs=0` would choose some default amount
of jobs, similar to `git -c fetch.parallel=0 fetch --multiple`. While our
documentation only ever promised that `fetch.parallel` would fall back to a
"sensible default", it makes sense to do the same for `--jobs`. So fall back
to online_cpus() and not BUG() out.
This fixes https://github.com/git-for-windows/git/issues/4302
Reported-by: Drew Noakes <drnoakes@microsoft.com>
Signed-off-by: Matthias Aßhauer <mha1993@live.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>