Improve update command (#159)

* Always try to update if asked for new inputs

If the user asks to re-input the cookiecutter context, it might try to
update some value. In that case, it is intersting to skip the
early return of the same template commit revision.

* Do not apply input changes to current template

If we reuse the same inputs for both current and new context
cruft will not be able to capture inputs changes, resulting in
less changes than the expected.

* Add option to refresh cookiecutter private variables

At this moment, it is not possible to update a cookiecutter private
variable with the update command because the current context
are not aware of the template changes.

Removing the private variables from cruft_state before creating
the updated context seems to refresh the private variables from
the template.

* Lint test

Co-authored-by: Sambhav Kothari <sambhavs.email@gmail.com>
Co-authored-by: Timothy Edmund Crosley <timothy.crosley@gmail.com>