Bug#748723: debian-installer: BTRFS installation aborts when formatting existing BTRFS partition

[Russell Coker <russell@coker.com.au>]

Package: debian-installer
Version: jessie_di_alpha_1
Severity: normal

http://cdimage.debian.org/cdimage/jessie_di_alpha_1/amd64/iso-cd/debian-jessie-DI-a1-amd64-netinst.iso

I just downloaded the Debian installer from the above URL and installed to
a BTRFS root filesystem on a partition that previously had BTRFS.  It aborted
and on virtual console 4 had the following messages:

partman: /dev/sda2 appears to contain an existing filesystem (btrfs).
partman: Error: Use the -f option to force overwrite.

I opened a shell and ran "cat /dev/zero > /dev/sda2" and after overwriting
the start of that partition I used the installer to successfully complete
that stage.

As the Debian installer had already asked me if I wanted to format the
partition I think it should either have used the -f option by default.