odb: rename repo_read_object_file()

Rename `repo_read_object_file()` to `odb_read_object()` to match other
functions related to the object database and our modern coding
guidelines.

Introduce a compatibility wrapper so that any in-flight topics will
continue to compile.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fmt-merge-msg.c

@@ -526,8 +526,8 @@ static void fmt_merge_msg_sigs(struct strbuf *out)
 		struct object_id *oid = origins.items[i].util;
 		enum object_type type;
 		unsigned long size;
-		char *buf = repo_read_object_file(the_repository, oid, &type,
-						  &size);
+		char *buf = odb_read_object(the_repository->objects, oid,
+					    &type, &size);
 		char *origbuf = buf;
 		unsigned long len = size;
 		struct signature_check sigc = { NULL };