merge-recursive: make !o->detect_rename codepath more obvious
Previously, if !o->detect_rename then get_renames() would return an
empty string_list, and then process_renames() would have nothing to
iterate over. It seems more straightforward to simply avoid calling
either function in that case.
Reviewed-by: Stefan Beller <sbeller@google.com> Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>