dir: stop using the index compatibility macros

dir: stop using the index compatibility macros

In order to make it clearer where the_index is being referenced, stop
using the index compatibility macros in dir.c.  This is to make it
easier to identify the functions which need to be convert to taking in a
'struct index_state' as a parameter.

The end goal would be to eliminate the need to reference global index
state in dir.c.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>