Add option to auto-use --jobs

Git 2.8.0 added the `--jobs` param to fetch submodules in parallel. E.g. `git submodule -q update --init --jobs 3 libs/config libs/headers libs/../tools/boost_install libs/../tools/build libs/../tools/cmake`

Would be great if boostdep had an option to auto-use that when possible. Checking the git version in Bash is a bit cumbersome and using `--git_args "--jobs 3"` is leading to errors for older git versions

Start by tracing boostdep's command-line handling for --git_args and the code that invokes git submodule update. Check how Git versions are detected or commands are assembled, then verify that --jobs is added when supported and that older Git versions continue to work without errors.

Written by the indexing model from the issue text.