This implements DiffToolsRegistry, which tracks all available diff

tools. It can be used to look up registered tool classes and instantiate

available tool instances (which will be cached for reuse).
New tools can also be registered dynamically, though this isn't

something that will be useful outside of very specialized custom code.
When looking up available tools, it will instantiate each tool, set it

up and check for availability on the system, and then cache it.
If an available tool can't be found, a MissingDiffToolError will be

raised. This will contain a list of all tools tried, and any

instructions that candidate tools may provide.

Unit tests pass.
Used this with in-progress changes to SCMClients.