merge-ort: add a special minimal index just for renormalization

merge-ort: add a special minimal index just for renormalization

renormalize_buffer() requires an index_state, which is something that
merge-ort does not operate with.  However, all the renormalization code
needs is an index with a .gitattributes file...plus a little bit of
setup.  Create such an index, along with the deallocation and
attr_direction handling.

A subsequent commit will add a function to finish the initialization
of this index.

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>