We still include "dir.h" in "reftable/system.h" even though it is not
used by anything but by a single unit test. Move it over into that unit
test so that we don't accidentally use any functionality provided by it
in the reftable codebase.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>