This updates Perforce, ClearCase, SOS, Subversion, and TFS to require

GNU Diff. These will be checked when a command starts up or when Python

code calls diff() on the client.
For now, we're explicitly requiring GNU Diff. Most of these will change

to allowing any tool, once everything has been updated appropriately.
This doesn't update the clients to use the new diff tool support yet. It

just sets the requirements.
check_gnu_diff() is now deprecated, and will be removed in RBTools 5.

Unit tests pass.
Tested a couple of these clients in test repositories, with and without

GNU Diff in the path. Verified that the commands failed if not found.