oidset: refactor oidset_insert_from_set()

oidset: refactor oidset_insert_from_set()

In a following commit, we will need to add all the oids from a set into
another set. In "list-objects-filter.c", there is already a static
function called add_all() to do that.

Let's rename this function oidset_insert_from_set() and move it into
oidset.{c,h} to make it generally available.

While at it, let's remove a useless `!= NULL`.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>