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/commands/models/ref.go

@@ -0,0 +1,9 @@
+package models
+
+type Ref interface {
+	FullRefName() string
+	RefName() string
+	ShortRefName() string
+	ParentRefName() string
+	Description() string
+}