submodule update: run at most one fetch job unless otherwise set

submodule update: run at most one fetch job unless otherwise set

In a028a1930c (fetching submodules: respect `submodule.fetchJobs`
config option, 2016-02-29), we made sure to keep the default
behavior of fetching at most one submodule at once when not setting
the newly introduced `submodule.fetchJobs` config.

This regressed in 90efe595c5 (builtin/submodule--helper: factor
out submodule updating, 2018-08-03). Fix it.

Reported-by: Sjon Hortensius <redacted>
Signed-off-by: Junio C Hamano <redacted>
Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>