merge-recursive: use STRING_LIST_INIT_NODUP

Initialize a string_list right when it's defined.  That's shorter, saves
a function call and makes it more obvious that we're using the NODUP
variant here.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>