upgraderepo: add a config option for parallel computation
The option is put to use to compute new copy tracing side data in parallel. It
use the multiprocessing module as it had the appropriate primitive for what we
needed. Gregory Szorc had concerned on windows so we disabled it there.
See inline comment for details on the parallel implementation.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.