largefiles: replace readstandin() by readasstandin() These code paths already (or should, for efficiency at repetition) know the target changectx and path of standin file.

largefiles: introduce readasstandin() to read hex hash from given filectx This will be used to centralize and encapsulate the logic to read hash from given (filectx of) standin file. readstandin() isn't suitable for this purpose, because there are some code paths, which want to read hex hash directly from filectx.