Extract a method CommitFilesContext.ReInit

Right now it doesn't do very much yet, but it's still worth it even in this
state, I'd say. The function is going to become a bit longer in the next commit,
though.

pkg/gui/controllers/switch_to_diff_files_controller.go

@@ -71,9 +71,8 @@ func (self *SwitchToDiffFilesController) enter(ref types.Ref) error {
 		}
 	}
 
+	commitFilesContext.ReInit(ref)
 	commitFilesContext.SetSelection(0)
-	commitFilesContext.SetRef(ref)
-	commitFilesContext.SetTitleRef(ref.Description())
 	commitFilesContext.SetCanRebase(canRebase)
 	commitFilesContext.SetParentContext(self.context)
 	commitFilesContext.SetWindowName(self.context.GetWindowName())