Originally from "Creating a svn.authorsfile when migrating from subversion to git", http://technicalpickles.com/posts/creating-a-svn-authorsfile-when-migrating-from-subversion-to-git/
Minor update, so the generated output would be legal for a svn.authorsfile right away.
Note, however, that you SHOULD nevertheless provide the authors' full names and email addresses for a clean migration, i.e.:
svn_username = Firstname Lastname