We had a lot of inconsistency between diff() function signatures in

SCMClients. Some took only a subset of the arguments. Some took

arguments not defined in BaseSCMClient.diff(). Git had its own

Git-specific argument, --git-find-renames-threshold.
This updates all function signatures to be compatible with the base

function signature.
The Git-specific option has been removed. Instead, GitClient.diff()

now just checks self.options, instead of rbt post and rbt diff

doing this.

Unit tests pass.
Tested rbt diff and rbt post with --git-find-renames-threshold.