Support rendering preview of template changes in a feature branch for `component update` / `package update`

[simu]

Context

We currently have no clean way to preview template changes on a feature branch of the template repository with Cruft. We can preview the changes by manually editing .cruft.json to point to the feature branch (assuming that the last rendered template commit is an ancestor of the feature branch). However, we don't have any way to preview the template changes in the sync GitHub action which runs for PRs on the template repository.

To support these previews out of the box with component update, we probably need to add additional logic to cruft._commands.update, as that command currently doesn't support using a different template version to render the "old" and "new" template states.

Additionally we'll have to add a command line flag which allows users to specify the feature branch that should be used.