Merge branch 'js/sparse-vs-split-index'

Mark in various places in the code that the sparse index and the
split index features are mutually incompatible.

* js/sparse-vs-split-index:
  split-index: it really is incompatible with the sparse index
  t1091: disable split index
  sparse-index: sparse index is disallowed when split index is active

diff --cc read-cache.c
Simple merge

diff --cc t/t1091-sparse-checkout-builtin.sh
Simple merge