I hope the number of commits by each contributor in the generated
AUTHORS file addresses (at least partially) John's concerns about
«the measure of contribution».

The new "build-aux/generate-authors.scm" file may be tested like so:

  guile ~/src/guix/build-aux/generate-authors.scm ~/src/guix /tmp/guix-authors


Not related: I wonder is there a reason to ignore (in .gitignore) the
whole "build-aux" directory?  IIUC this led to commit 8bc35f8, right?