Re: [PATCH 2/2] shortlog: introduce `--email-only` to only show emails

> When a shortlog caller wants to group output by, say, author email, they
> can easily express this with:
> 
>     $ git shortlog --group=format:%ae
> 
> and restrict output to specific email(s) with the new `--group-filter`
> option introduced by the previous commit.
> 
> But they are not able to apply the same treatment to identities that
> appear in trailers. Doing:
> 
>     $ git shortlog -e --group=format:%ae --group=trailer:Co-authored-by
> 
> will produce funky results, interspersing proper emails with full "Name
> <email>" identities from the Co-authored-by trailer (or anything else
> that might appear there), like:
> 
>     461  me@ttaylorr.com
>      11  Taylor Blau <me@ttaylorr.com>
> 
> So if the caller wants to restrict output to a set of matching email
> addresses (say, "me@ttaylorr.com"), they cannot do it with a
> `--group-filter`, since it would discard the group "Taylor Blau
> <me@ttaylorr.com>".
> 
> Introduce a new `--email-only` option, which extracts the email
> component of an identity from all shortlog groups, including trailers.
> It behaves similarly to the `-e` option, but replaces its output with
> just the email component, instead of adding it on to the end.
> 
> Now, `shortlog` callers can perform:
> 
>     $ git shortlog -s --group=author --group=trailer:Co-authored-by \
>         --email-only --group-filter="<me@ttaylorr.com>"
>        472  <me@ttaylorr.com>
> 
> to obtain the output they want.