diffcore-order: fix leaking buffer when parsing orderfiles

diffcore-order: fix leaking buffer when parsing orderfiles

In `prepare_order()` we parse an orderfile and assign it to a global
array. In order to save on some allocations, we replace newlines with
NUL characters and then assign pointers into the allocated buffer to
that array. This can cause the buffer to be completely unreferenced
though in some cases, e.g. because the order file is empty or because we
had to use `xmemdupz()` to copy the lines instead of NUL-terminating
them.

Refactor the code to always `xmemdupz()` the strings. This is a bit
simpler, and it is rather unlikely that saving a handful of allocations
really matters. This allows us to release the string buffer and thus
plug the memory leak.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>