odb: use enum for `odb_write_object` flags

odb: use enum for `odb_write_object` flags

We've got a couple of functions that accept `odb_write_object()` flags,
but all of them accept the flags as an `unsigned` integer. In fact, we
don't even have an `enum` for the flags field.

Introduce this `enum` and adapt functions accordingly according to our
coding style.

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