object-store.h: move for_each_alternate_ref() from transport.h

object-store.h: move for_each_alternate_ref() from transport.h

There's nothing inherently transport-related about enumerating the
alternate ref tips. The code has lived in transport.[ch] because the
only use so far had been advertising available tips during transport.
But it could be used for more, and a future patch will teach rev-list to
access these refs.

Let's move it alongside the other alt-odb code, declaring it in
object-store.h with the implementation in sha1-file.c.

This lets us drop the inclusion of transport.h from receive-pack, which
perhaps shows how it was misplaced (though receive-pack is about
transporting objects, transport.h is mostly about the client side).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>