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>