sha1_file: rename LOOKUP_REPLACE_OBJECT

sha1_file: rename LOOKUP_REPLACE_OBJECT

The LOOKUP_REPLACE_OBJECT flag controls whether the
lookup_replace_object() function is invoked by
sha1_object_info_extended(), read_sha1_file_extended(), and
lookup_replace_object_extended(), but it is not immediately clear which
functions accept that flag.

Therefore restrict this flag to only sha1_object_info_extended(),
renaming it appropriately to OBJECT_INFO_LOOKUP_REPLACE and adding some
documentation. Update read_sha1_file_extended() to have a boolean
parameter instead, and delete lookup_replace_object_extended().

parse_sha1_header() also passes this flag to
parse_sha1_header_extended() since commit 46f0344 ("sha1_file: support
reading from a loose object of unknown type", 2015-05-03), but that has
had no effect since that commit. Therefore this patch also removes this
flag from that invocation.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>