The empty treeish in ":path" means "index". This is actually a special
case of the ":stage:path" syntax where it is documented, but mentioning
it also together with "treeish:path" is helpful, so do it.
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>