largefiles: make copytostore() accept only changectx as the 2nd argument (API) As the name describes, the 2nd argument 'revorctx' of copytostore() can accept non-changectx value, for historical reason, But, since 10561eb97c7f, copyalltostore(), the only one copytostore() client in Mercurial source tree, always passes changectx as 'revorctx'. Therefore, it is reasonable to make copytostore() accept only changectx as the 2nd argument, now.