git-add: remove conflicting entry when adding.

git-add: remove conflicting entry when adding.

When replacing an existing file A with a directory A that has a
file A/B in it in the index, 'git add' did not succeed because
it forgot to pass the allow-replace flag to add_cache_entry().

It might be safer to leave this as an error and require the user
to explicitly remove the existing A first before adding A/B
since it is an unusual case, but doing that automatically is
much easier to use.

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