odb: introduce odb_write_object()

We do not have a backend-agnostic way to write objects into an object
database. While there is `write_object_file()`, this function is rather
specific to the loose object format.

Introduce `odb_write_object()` to plug this gap. For now, this function
is a simple wrapper around `write_object_file()` and doesn't even use
the passed-in object database yet. This will change in subsequent
commits, where `write_object_file()` is converted so that it works on
top of an `odb_source`. `odb_write_object()` will then become
responsible for deciding which source an object shall be written to.

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

builtin/mktree.c

@@ -63,7 +63,7 @@ static void write_tree(struct object_id *oid)
 		strbuf_add(&buf, ent->oid.hash, the_hash_algo->rawsz);
 	}
 
-	write_object_file(buf.buf, buf.len, OBJ_TREE, oid);
+	odb_write_object(the_repository->objects, buf.buf, buf.len, OBJ_TREE, oid);
 	strbuf_release(&buf);
 }