This introduces a new BaseSCMClient.requires_diff_tool attribute,

which can be set to indicate the diff() method's requirements.
To indicate that any available diff tool can be used, this should be set

to True.
To indicate that a particular diff tool or tools are required, this can be

set to a list of diff tool IDs.
To indicate that a tool diff is not needed (if, for example, the SCM's

native diff tool is used), this can remain at the default of False.
A new BaseSCMClient.get_diff_tool() method is used to attempt to fetch

the right diff tool based on this setting. It will raise a

MissingDiffToolError if one is unavailable.

Unit tests pass.
Tested with in-progress changes making use of this new functionality.