Multithread conflict detection

Allow for the option of using multiple threads to check for conflicts.

This requires that each thread have it's own copy of the repository, so there is
a space:performance tradeoff here.

It is also recommended to store the repositories on different physical volumes
and symlink them into the local repo directory to avoid disk thrashing.
Update tests
Merge pull request #99 from rschlaikjer/multithread-conflict-detection

Multithread conflict detection

Give more workers to the merge conflict detection system to parallelize the work.
Each worker owns its own workspace which has a copy of the main repository. Nothing is shared at the moment.