Run source and target build in parallel if projectDir is different

When doing a build comparison where source and target build have different projectDirs set, they should not disturb each other, so it should be possible to run those two builds in parallel. This should be configurable on the task though with a default value of enabled. It could be that some builds relay on some other shared resource which makes parallel execution impossible. But for the majority of cases, I think the parallelisation should be possible.