add-apt-repository doesn't add ppa if file with comments exists

[Javier Collado]

add-apt-repository fails to add a ppa if the file for the ppa already exists with the deb entries commented out. Besides this, it doesn't print any error message and returns success (0).

One way to reproduce it is as follows:
- Add PPA to sources
$ sudo add-apt-repository -y ppa:javier.collado/ppa

$ cat /etc/apt/sources.list.d/javier_collado-ppa-quantal.list
deb http://ppa.launchpad.net/javier.collado/ppa/ubuntu quantal main
deb-src http://ppa.launchpad.net/javier.collado/ppa/ubuntu quantal main

- Remove PPA from sources
$ sudo ppa-purge ppa:javier.collado/ppa

$ cat /etc/apt/sources.list.d/javier_collado-ppa-quantal.list
# deb http://ppa.launchpad.net/javier.collado/ppa/ubuntu quantal main
# deb-src http://ppa.launchpad.net/javier.collado/ppa/ubuntu quantal main

- Add again PPA to sources
$ sudo add-apt-repository -y ppa:javier.collado/ppa
Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /tmp/tmp.Kekxq78ff1 --trustdb-name /etc/apt//trustdb.gpg --keyring /etc/apt/trusted.gpg --primary-keyring /etc/apt/trusted.gpg --keyserver hkp://keyserver.ubuntu.com:80/ --recv 36AA079422DB186A78174271FFD1BF24CF31078F
gpg: requesting key CF31078F from hkp server keyserver.ubuntu.com
gpg: key CF31078F: "Launchpad PPA for Javier Collado" not changed
gpg: Total number processed: 1
gpg: unchanged: 1

- PPA wasn't really added
$ cat /etc/apt/sources.list.d/javier_collado-ppa-quantal.list
# deb http://ppa.launchpad.net/javier.collado/ppa/ubuntu quantal main
# deb-src http://ppa.launchpad.net/javier.collado/ppa/ubuntu quantal main

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Launchpad Janitor (janitor)]

This bug was fixed in the package software-properties - 0.92.5

---------------
software-properties (0.92.5) quantal; urgency=low

  [ Colin Watson ]
  * add-apt-repository:
    - Force output encoding to UTF-8 even in non-UTF-8 locales
      (LP: #1041431).

  [ sampo555 ]
  * Construct the path of a PPA source file properly (LP: #972617, #994515,
    #1018327, #1037916).
 -- Colin Watson <email address hidden> Mon, 17 Sep 2012 14:41:12 +0100