Make read-cache.c "the_index" free.

Make read-cache.c "the_index" free.

This makes all low-level functions defined in read-cache.c to
take an explicit index_state structure as their first parameter,
to specify which index to work on.  These functions
traditionally operated on "the_index" and were named foo_cache();
the counterparts this patch introduces are called foo_index().

The traditional foo_cache() functions are made into macros that
give "the_index" to their corresponding foo_index() functions.

Signed-off-by: Junio C Hamano <junkio@cox.net>