add-apt-repository should have an option for adding only binary DEBs (without DEB-SRC)

[Tugdual de Kerviler]

STEPS TO REPRODUCE:
  1. launch terminal
  2. add some repository (for example, proprietary Opera without source code) with
   sudo add-apt-repository 'deb http://deb.opera.com/opera/ stable non-free'
  3. update repositories with
   sudo apt-get update

EXPECTED RESULTS:
  Index files are updated, opera package may be installed with
    sudo apt-get install opera

ACTUAL RESULTS:
  Got error messages:
   W: Failed to fetch http://deb.opera.com/opera/dists/stable/Release Unable to find expected entry 'non-free/source/Sources' in Release file (Wrong sources.list entry or malformed file)

    E: Some index files failed to download. They have been ignored, or old ones used instead.

  Unable to install opera with
    sudo apt-get install opera
  Able to install opera after commenting out deb-src line.

Please add option to add-apt-repository for skipping deb-src line.

[Launchpad Janitor (janitor)]

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