The empty state now only gets shown when no files exist in the branch.
If the user is reviewing 2 versions with no files, we don't show the state.

Refactors the diff app spec to use Vue test utils.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/48635

Rename btn-new, btn-create and btn-save to btn-success

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>