When users are using `git update-index --(no-)split-index`, they
may expect the split-index feature to be used or not according to
the option they just used, but this might not be the case if the
new "core.splitIndex" config variable has been set. In this case
let's warn about what will happen and why.
Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>