Move the Ref interface from gui/types to models

This is a type that can be useful for model/backend stuff, so move it there. We
are going to use it in the API of the commit loader.

pkg/gui/controllers/helpers/sub_commits_helper.go

@@ -24,7 +24,7 @@ func NewSubCommitsHelper(
 }
 
 type ViewSubCommitsOpts struct {
-	Ref                     types.Ref
+	Ref                     models.Ref
 	RefToShowDivergenceFrom string
 	TitleRef                string
 	Context                 types.Context