Extend `checkout` command to filter impacts

At the moment it is very simple, reset the index to exactly what it was at a given `RefSpec`. The git equivalent allows you to pass a list of files to touch. The equivalent here of letting you undo changes to specific budgets would be nice.

Start by locating the checkout command and its RefSpec handling, then read how budgets are identified and reset. Define the expected command syntax and behavior for selecting specific budgets; done means checkout can undo only the requested budgets while preserving the rest.

Written by the indexing model from the issue text.