From: =?UTF-8?q?Matthias=20A=C3=9Fhauer?= <mha1993@live.de>

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>

Submitted-As: https://lore.kernel.org/git/pull.1483.git.1676928805555.gitgitgadget@gmail.com