read-tree: Fix regression with creation of a new index file.

read-tree: Fix regression with creation of a new index file.

Reading the index into an empty file has been broken by
5a56da58060e50980fab0f4c38203a25440d1530, since it causes the existing
index to always be loaded first, and dies if it's an empty file:

$ GIT_INDEX_FILE=`mktemp` git read-tree master
fatal: index file smaller than expected

It breaks for instance committing from git.el. This patch reverts to the
previous behavior of only loading the index when merging it.

Signed-off-by: Alexandre Julliard <julliard@winehq.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>