changelog entry for MP with multiple authors should summarize as [ author1, author2 [, ...] ], not repeat changelog for each author

[Steve Langasek]

when an MP includes commits by multiple authors, the changelog generated by update_changelog looks like this:

 autopilot (1.5.1+15.04.20150522-0ubuntu1) vivid; urgency=medium

   [ CI Train Bot ]
   * New rebuild forced.

   [ Christopher Lee ]
   * Bug fixes for logging (debug level now -vv) and application of CPO
     bases for proxy objects. (LP: #1425721, #1376996, #1420949)

   [ Federico Gimenez ]
   * Bug fixes for logging (debug level now -vv) and application of CPO
     bases for proxy objects. (LP: #1425721, #1376996, #1420949)

   [ Leo Arias ]
   * Bug fixes for logging (debug level now -vv) and application of CPO
     bases for proxy objects. (LP: #1425721, #1376996, #1420949)

   [ Richard Huddie ]
   * Bug fixes for logging (debug level now -vv) and application of CPO
     bases for proxy objects. (LP: #1425721, #1376996, #1420949)

   [ Vincent Ladeuil ]
   * Bug fixes for logging (debug level now -vv) and application of CPO
     bases for proxy objects. (LP: #1425721, #1376996, #1420949)

This is unnecessarily redundant. Instead, the changelog should look like this:

 autopilot (1.5.1+15.04.20150522-0ubuntu1) vivid; urgency=medium

   [ CI Train Bot ]
   * New rebuild forced.

   [ Christopher Lee, Federico Gimenez, Leo Arias, Richard Huddie, Vincent Ladeuil ]
   * Bug fixes for logging (debug level now -vv) and application of CPO
     bases for proxy objects. (LP: #1425721, #1376996, #1420949)

An example of such an MP can be found here:

  https://code.launchpad.net/~autopilot/autopilot/trunk/+merge/258580