Set groundwork for better disabled reasons with range select

Something dumb that we're currently doing is expecting list items
to define an ID method which returns a string. We use that when copying
items to clipboard with ctrl+o and when getting a ref name for diffing.

This commit gets us a little deeper into that hole by explicitly requiring
list items to implement that method so that we can easily use the new
helper functions in list_controller_trait.go.

In future we need to just remove the whole ID thing entirely but I'm too
lazy to do that right now.

pkg/gui/controllers/basic_commits_controller.go

@@ -16,6 +16,7 @@ type ContainsCommits interface {
 	types.Context
 	types.IListContext
 	GetSelected() *models.Commit
+	GetSelectedItems() ([]*models.Commit, int, int)
 	GetCommits() []*models.Commit
 	GetSelectedLineIdx() int
 }
@@ -36,6 +37,7 @@ func NewBasicCommitsController(c *ControllerCommon, context ContainsCommits) *Ba
 			c,
 			context,
 			context.GetSelected,
+			context.GetSelectedItems,
 		),
 	}
 }