getncchanges.c: Refactor how we add ancestor links

getncchanges.c: Refactor how we add ancestor links

If the current object had already been sent as an ancestor, we were
duplicating the code that added its links and updated the HWM mark.
We want these to occur when we reach the place where the object's USN
naturally occurs.

Instead of duplicating this code, we can just skip the call to
get_nc_changes_build_object() if the object has already been sent.
There is already an existing 'nothing to send'/continue case after we've
updated the highwater mark.

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>