for_each_*_object: take flag arguments as enum

for_each_*_object: take flag arguments as enum

It's not wrong to pass our flags in an "unsigned", as we
know it will be at least as large as the enum.  However,
using the enum in the declaration makes it more obvious
where to find the list of flags.

While we're here, let's also drop the "extern" noise-words
from the declarations, per our modern coding style.

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