shortlog: implement `--group=author` in terms of `--group=<format>`

shortlog: implement `--group=author` in terms of `--group=<format>`

Instead of handling SHORTLOG_GROUP_AUTHOR separately, reimplement it as
a special case of the new `--group=<format>` mode, where the author mode
is a shorthand for `--group='%aN <%aE>'.

Note that we still need to keep the SHORTLOG_GROUP_AUTHOR enum since it
has a different meaning in `read_from_stdin()`, where it is still used
for a different purpose.

Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>