sequencer: implement apply_autostash_oid()

sequencer: implement apply_autostash_oid()

Split apply_save_autostash() into apply_autostash_oid() and
apply_save_autostash() where the former operates on an OID string and
the latter reads the OID from a file before passing it into
apply_save_autostash_oid().

This function is required for a future commmit which will rely on being
able to apply an autostash whose OID is stored as a string.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>