have setConcurrency stop any running git coprocesses

have setConcurrency stop any running git coprocesses

When non-concurrent git coprocesses have been started, setConcurrency
used to not stop them, and so could leak processes when enabling
concurrency, eg when forkState is called.

I do not think that ever actually happened, given where setConcurrency
is called. And it probably would only leak one of each process, since it
never downgrades from concurrent to non-concurrent.