checkout: add spice.checkout.verbose config

[abhinav]

This adds a new configuration option that controls whether checkout
navigation commands print verbose messages when switching branches.
The option defaults to true but allows users to opt out.

Resolves #717

[abhinav]

This change is part of the following stack:

• checkout: add spice.checkout.verbose config #753 ◀

_{Change managed by git-spice.}