getncchanges.c: Rename anc_cache to obj_cache

getncchanges.c: Rename anc_cache to obj_cache

When we add GET_TGT support we will reuse the ancestor cache and it
should work the same way - if we've already sent an object because it
was needed for resolving a child object or a link target, then there's
no point sending it again.

This just renames anc_cache --> obj_cache.

An extra is_get_anc flag has been added to getnc_state - once GET_TGT
support is added, we can't assume GET_ANC based solely on the existence
of the obj_cache.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>