Post conflict is not detected

[ralsina]

To reproduce:

nikola init foo
cd foo
nikola new_post -t foo posts/foo.txt
nikola new_post -t foo posts/foo.rst

Both posts should try to create output/posts/foo.html and not only does this not fail or cause a conflict, but there is a check in scan_posts (the seen set) that should see it and doesn't.

The timeline only contains the .txt version of the post. If I remove the .txt file, then it will only contain the .rst one.