Returns a diff as a string from the current HEAD to the given commit.

def diff_commit(self, commit, compare_point=None):
        """
        Returns a diff as a string from the current HEAD to the given commit.
        """
        # @@@ This is a security hazard as compare-point is user-passed in
        # data. Doesn't matter until we wrap this in a service.
        if compare_point is not None:
            self.apply_commit(compare_point)
        return self.executor("cd %s && git diff %s" % (self.dirname, commit))