This introduces Command.needs_diffs. If set to True, SCMClient

instantiation will check if any required diff tools are available. This

will allow the command to fail early with useful instructions.
This is set on rbt diff and rbt post.
Note that this does mean that rbt post can't be used if the matching

client doesn't have an available diff tool. While it's technically

possible to use rbt post without generating diffs, this isn't the

common case, and an available diff tool is a pretty reasonable

expectation to set.

Unit tests pass.
Tested rbt diff and rbt post with and without a working diff tool

for a client that requires it (Perforce, in my case), when used with an

upcoming change that adds these requirements to each client.