src[0] points to the index entry in the merge case and to the first
tree to unpack in the non-merge case. We only want to mark the index
entry, so check first if we're merging.
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx> Signed-off-by: Junio C Hamano <gitster@pobox.com>