sha1_file: convert check_sha1_signature to struct object_id

sha1_file: convert check_sha1_signature to struct object_id

Convert this function to take a pointer to struct object_id and rename
it check_object_signature.  Introduce temporaries to convert the return
values of lookup_replace_object and lookup_replace_object_extended into
struct object_id.

The temporaries are needed because in order to convert
lookup_replace_object, open_istream needs to be converted, and
open_istream needs check_sha1_signature to be converted, causing a loop
of dependencies.  The temporaries will be removed in a future patch.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>