Fix switching to a branch with D/F when current branch has file D.

Fix switching to a branch with D/F when current branch has file D.

This loosens the over-eager verify_absent() check that gets
upset to find directory D in the current working tree when
switching to a branch that has a file there.  The check needs to
make sure that we do not lose precious working tree files as a
result of removing directory D and replacing it with the file
from the other branch, which is a tad expensive but this is a
less common case.

Signed-off-by: Junio C Hamano <junkio@cox.net>