fetch: let --jobs=<n> parallelize --multiple, too

fetch: let --jobs=<n> parallelize --multiple, too

So far, `--jobs=<n>` only parallelizes submodule fetches/clones, not
`--multiple` fetches, which is unintuitive, given that the option's name
does not say anything about submodules in particular.

Let's change that. With this patch, also fetches from multiple remotes
are parallelized.

For backwards-compatibility (and to prepare for a use case where
submodule and multiple-remote fetches may need different parallelization
limits), the config setting `submodule.fetchJobs` still only controls
the submodule part of `git fetch`, while the newly-introduced setting
`fetch.parallel` controls both (but can be overridden for submodules
with `submodule.fetchJobs`).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>