Merge branch 'sb/submodule-fetchjobs-default-to-one'

"git submodule update" ought to use a single job unless asked, but
by mistake used multiple jobs, which has been fixed.

* sb/submodule-fetchjobs-default-to-one:
submodule update: run at most one fetch job unless otherwise set

diff --cc builtin/submodule--helper.c
Simple merge