[RevisionMachine] Add multi process support for DiffTool

[daxenberger]

Originally reported on Google Code with ID 58

The DiffTool has to process several source files (currently up to 120 for enwiki), which
are independent.
At the moment, it is possible to create several config files - each configuring the
processing of a distinct set of source files.
The config files can then be used to start several instances of the DiffTool.

We should now add a feature that allows the user to configure the number of processes
that should be used to process the source files (without the need of individual config
files for each process).
The DiffTool should then divide the source files among the processes and create a separate
instance of the DiffTool_Thread.

This not really a multithread-solution - but better than nothing.

Reported by oliver.ferschke on 2011-09-14 09:14:05