sha1_file: rename LOOKUP_UNKNOWN_OBJECT

sha1_file: rename LOOKUP_UNKNOWN_OBJECT

The LOOKUP_UNKNOWN_OBJECT flag was introduced in commit 46f0344
("sha1_file: support reading from a loose object of unknown type",
2015-05-03) in order to support a feature in cat-file subsequently
introduced in commit 39e4ae3 ("cat-file: teach cat-file a
'--allow-unknown-type' option", 2015-05-03). Despite its name and
location in cache.h, this flag is used neither in
read_sha1_file_extended() nor in any of the lookup functions, but used
only in sha1_object_info_extended().

Therefore rename this flag to OBJECT_INFO_ALLOW_UNKNOWN_TYPE, taking the
name of the cat-file flag that invokes this feature, and move it closer
to the declaration of sha1_object_info_extended(). Also add
documentation for this flag.

OBJECT_INFO_ALLOW_UNKNOWN_TYPE is defined to 2, not 1, to avoid
conflicting with LOOKUP_REPLACE_OBJECT. Avoidance of this conflict is
necessary because sha1_object_info_extended() supports both flags.

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