This change updates the parse_revision_spec, diff, and

create_commit methods for SCMClients to avoid using mutable lists as

defaults for arguments. These are now set to use

(Sequence[str] | None) = None, and initialize the values in the body

of the method instead.
Because all "real" call sites pass these in (as empty lists), I've

added deprecation warnings that we will be removing the default value

(and the | None) in rbtools 8.

Ran unit tests.