For some reason, the git-client plugin iterates through the list of submodules (presumably by parsing the .gitmodules file) and manually issues a git submodule update command for each. Instead of this, it should simply issue a single git submodule update command without specifying a particular submodule, and then let the CLI do its job.

As part of this change, the new threads property of SubmoduleUpdateCommand should just be passed as --jobs rather than manually spawning threads in Java/Groovy.