Merge branch 'tb/shortlog-group'

Merge branch 'tb/shortlog-group'

"git shortlog" learned to group by the "format" string.

* tb/shortlog-group:
  shortlog: implement `--group=committer` in terms of `--group=<format>`
  shortlog: implement `--group=author` in terms of `--group=<format>`
  shortlog: extract `shortlog_finish_setup()`
  shortlog: support arbitrary commit format `--group`s
  shortlog: extract `--group` fragment for translation
  shortlog: make trailer insertion a noop when appropriate
  shortlog: accept `--date`-related options