Merge branch 'sb/submodule-parallel-update'

A major part of "git submodule update" has been ported to C to take advantage of the recently added framework to run download tasks in parallel. * sb/submodule-parallel-update: clone: allow an explicit argument for parallel submodule clones submodule update: expose parallelism to the user submodule helper: remove double 'fatal: ' prefix git submodule update: have a dedicated helper for cloning run_processes_parallel: rename parameters for the callbacks run_processes_parallel: treat output of children as byte array submodule update: direct error message to stderr fetching submodules: respect `submodule.fetchJobs` config option submodule-config: drop check against NULL submodule-config: keep update strategy around