preload-index: update GIT_FORCE_PRELOAD_TEST support
Rename GIT_FORCE_PRELOAD_TEST to GIT_TEST_PRELOAD_INDEX for consistency with
the other GIT_TEST_ special setups and properly document its use.
Add logic in t/test-lib.sh to give a warning when the old variable is set to
let people know they need to update their environment to use the new
variable.
Signed-off-by: Ben Peart <Ben.Peart@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>