rev-parse: add --shared-index-path to get shared index path

rev-parse: add --shared-index-path to get shared index path

Normally scripts do not have to be aware about split indexes because
all shared indexes are in $GIT_DIR. A simple "mv $tmp_index
$GIT_DIR/somewhere" is enough. Scripts that generate temporary indexes
and move them across repos must be aware about split index and copy
the shared file as well. This option enables that.

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