archimport: remove files from the index before adding/updating

This fixes a bug when importing where a directory gets removed/renamed
but is immediately replaced by a file of the same name in the same
changeset.

This fix only applies to the accurate (default) strategy the moment.

This patch should also fix the fast strategy if/when it is updated
to handle the cases that would've triggered this bug.

This bug was originally found in git-svn, but I remembered I did the
same thing with archimport as well.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>