object-file API: pass an enum to read_object_with_reference()

Change the read_object_with_reference() function to take an "enum
object_type". It was not prepared to handle an arbitrary "const
char *type", as it was itself calling type_from_string().

Let's change the only caller that passes in user data to use
type_from_string(), and convert the rest to use e.g. "OBJ_TREE"
instead of "tree_type".

The "cat-file" caller is not on the codepath that
handles"--allow-unknown", so the type_from_string() there is safe. Its
use of type_from_string() doesn't functionally differ from that of the
pre-image.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1561,7 +1561,7 @@ int cache_name_stage_compare(const char *name1, int len1, int stage1, const char
 
 void *read_object_with_reference(struct repository *r,
 				 const struct object_id *oid,
-				 const char *required_type,
+				 enum object_type required_type,
 				 unsigned long *size,
 				 struct object_id *oid_ret);