worker: allow a function to be run in multiple worker processes
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.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state