use repo_get_oid_with_flags()

use repo_get_oid_with_flags()

get_oid_with_context() allows specifying flags and reports object
details via a passed-in struct object_context.  Some callers just want
to specify flags, but don't need any details back.  Convert them to
repo_get_oid_with_flags(), which provides just that and frees them from
dealing with the context structure.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>