shortlog: support arbitrary commit format `--group`s

shortlog: support arbitrary commit format `--group`s

In addition to generating a shortlog based on committer, author, or the
identity in one or more specified trailers, it can be useful to generate
a shortlog based on an arbitrary commit format.

This can be used, for example, to generate a distribution of commit
activity over time, like so:

    $ git shortlog --group='%cd' --date='format:%Y-%m' -s v2.37.0..
       117  2022-06
       274  2022-07
       324  2022-08
       263  2022-09
         7  2022-10

Arbitrary commit formats can be used. In fact, `git shortlog`'s default
behavior (to count by commit authors) can be emulated as follows:

    $ git shortlog --group='%aN <%aE>' ...

and future patches will make the default behavior (as well as
`--committer`, and `--group=trailer:<trailer>`) special cases of the
more flexible `--group` option.

Note also that the SHORTLOG_GROUP_FORMAT enum value is used only to
designate that `--group:<format>` is in use when in stdin mode to
declare that the combination is invalid.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>