Fixes and improvements for changelog auto-generation

The script generated a broken changelog if the current commit is tagged as
a release. That works now.

Additional improvements:
* Merge commits are ignored
* "^fixup!" commits are ignored
* Commit summaries are line-wrapped
* Authors can be merged by manually supplied name mapping
* The first release also includes a list of commits. They were originally
  excluded because the first release contains so many commits, including
  many unclean ones. But excluding those commits also means excluding the
  attribution, so it is not really an option.
* Authors are ordered by the number of their contributions in a release
  to make occasional contributors more visible.

(cherry picked from commit 328caf00)