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.
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..