"git shortlog" has been taught to group commits by the contents of
the trailer lines, like "Reviewed-by:", "Coauthored-by:", etc.

* jk/shortlog-group-by-trailer:
  shortlog: allow multiple groups to be specified
  shortlog: parse trailer idents
  shortlog: rename parse_stdin_ident()
  shortlog: de-duplicate trailer values
  shortlog: match commit trailers with --group
  trailer: add interface for iterating over commit trailers
  shortlog: add grouping option
  shortlog: change "author" variables to "ident"