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.
rebase: abort hg pull --rebase if rebase.requiredest is set (
issue5514)
Previously, the pull would succeed, but the subsequent rebase would fail due
to the rebase.requiredest flag. Now abort earlier with a more useful error
message.