CVS and SVN now change to the root directory of the repository for

performing diffs (before changing back).
Add the _get_repository_root method to CVSClient that determines

the root directory of the current CVS repository. It does this by

reading the CVS/Repository file in the current directory and

determining the working direcotry's depth relative to the CVSRoot;

that is the number of directories to traverse up to find the

repository root.
The base_dir parameter of all rbtools.utils.diffs method is no

longer optional because all SCMTools that utilize it require

specifying the root of the repository.

Unit tests pass.

The diffs are generated correctly.