largefiles: remove unused readstandin() Now, there is no client of readstandin().

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.

largefiles: remove unused keyword argument of copytostore() (API) AFAIK, 'uploaded' argument of copytostore() (or copytocache(), before renaming at c65f5b6e26d4) has been never used both on caller and callee sides, since official release of bundled largefiles extension.

largefiles: add copytostore() fstandin argument to replace readstandin() (API) copyalltostore(), only one caller of copytostore(), already knows standin file name of the target largefile. Therefore, passing it to copytostore() is more efficient than calculating it in copytostore() or readstandin().