builtin-clone: fix initial checkout

builtin-clone: fix initial checkout

Somewhere in the process of finishing up builtin-clone, the update of
the working tree was lost.  This was due to not using the option "merge"
for unpack_trees().

Breakage noticed by Kevin Ballard.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Tested-by: Jeff King <peff@peff.net>
Acked-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>