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