ref_remove_duplicates(): simplify loop logic

ref_remove_duplicates(): simplify loop logic

Change the loop body into the more straightforward

* remove item from the front of the old list
* if necessary, add it to the tail of the new list

and return a pointer to the new list (even though it is currently
always the same as the input argument, because the first element in
the list is currently never deleted).

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>