If a large update triggered an abort, it was possible for the main
thread to still update the dirstate.
This fix is incomplete, as the failing worker now doesn't generate a
proper error message. This is difficult in the fork-based framework,
which relies on exceptions propagating to the top of the dispatcher
for formatting.
We now wait for worker processes in a separate thread, so that we can
spot failures in a timely way, wihout waiting for the progress pipe
to drain.
If a worker fails, we recover the pre-parallel-update behaviour of
failing early by killing its peers before propagating the failure.
If we estimate that it will be worth the cost, we run the function in
multiple processes. Otherwise, we run it in-process.
Children report progress to the parent through a pipe.
Not yet implemented on Windows.
