Wrong paper built if author's fork's branch contains more than one paper

[cbcunc]

Sometimes authors merge the canonical branch into their scipy_proceedings fork's branch. When they do so, builder.BuildManager.build_paper() builds the wrong paper. This is because the BuildManager.paper property manager incorrectly assumes that the author's paper is the only unexcluded paper in the author's fork's branch:

return papers[0].split('/')[-1]

Determine another method for selecting the author's paper from the author's branch and implement it into builder.BuildManager.