renaming files doesn't check for name collisions within the torrent

[pcordes]

Renaming files within a torrent lets you rename two files within the same torrent to the same name. A rename(2) system call apparently happens, because the first file with that name was gone. After renaming the two entries to separate names again, one of the files was still gone.

I hit this bug with the GTK UI using git commit 0278e782e0f044d8184d92ef6f6464eb8e1c661c (Wed May 11 16:08:03 2016 +0100) on AMD64 Ubuntu.

This can be checked without even looking at the filesystem, so the GUI should probably check at least within the current torrent for conflicts.