do not overwrite untracked during merge from unborn branch

In case HEAD does not point to a valid commit yet, merge is
implemented as a hard reset. This will cause untracked files to be
overwritten.

Instead, assume the empty tree for HEAD and do a regular merge. An
untracked file will cause the merge to abort and do nothing. If no
conflicting files are present, the merge will have the same effect
as a hard reset.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>