read-cache: force split index mode with GIT_TEST_SPLIT_INDEX

read-cache: force split index mode with GIT_TEST_SPLIT_INDEX

This could be used to run the whole test suite with split
indexes. Index splitting is carried out at random. "git read-tree"
also resets the index and forces splitting at the next update.

I had a lot of headaches with the test suite, which proves it
exercises split index pretty good.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>