Rename add_file_to_index() to add_file_to_cache()

Rename add_file_to_index() to add_file_to_cache()

This function was not called "add_file_to_cache()" only because
an ancient program, update-cache, used that name as an internal
function name that does something slightly different.  Now that
is gone, we can take over the better name.

The plan is to name all functions that operate on the default
index xxx_cache().  Later patches create a variant of them that
take an explicit parameter xxx_index(), and then turn
xxx_cache() functions into macros that use "the_index".

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