unpack-trees: remove redundant path search in verify_absent

unpack-trees: remove redundant path search in verify_absent

Since the only caller, verify_absent, relies on the fact that o->pos
points to the next index entry anyways, there is no need to recompute
its position.

Furthermore, if a nondirectory entry were found, this would return too
early, because there could still be an untracked directory in the way.
This is currently not a problem, because verify_absent is only called
if the index does not have this entry.

Signed-off-by: Junio C Hamano <gitster@pobox.com>