We have a unit test that spies on GNUDiffTool.check_available() to

simulate a result, and this ended up leaking state into other tests,

since instances of diff tools are cached by the registry.
This change implements the ability to reset the registry. This is

currently done in only the one test that leaks state at the moment,

though potentially we'd consider it worthwhile to do this on test

teardown later.
It also changes the caching mechanism used for

BaseSCMClient.get_diff_tool() to just store a local copy on the

instance, as @lru_cache can cause memory leaks when used on class

methods.

Unit tests pass.
Tested this with upcoming changes, which were sensitive to this state

leak.