streaming: inline call to read_object_file_extended()

streaming: inline call to read_object_file_extended()

The open_istream_incore() function is the only direct user of
read_object_file_extended(), and the only caller which unsets the
lookup_replace flag. Since read_object_file_extended() is now just a
thin wrapper around oid_object_info_extended(), let's inline the call.
That will let us simplify read_object_file_extended() in the next patch.

The inlined version here is a few more lines because of the query setup,
but it's much more flexible, since we can pass (or omit) any flags we
want.

Note the updated comment in the istream struct definition. It was
already slightly wrong (we never called read_object(); it has been
read_object_file_extended() since day one), but should now be accurate.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>