shortlog: add grouping option

shortlog: add grouping option

In preparation for adding more grouping types, let's refactor the
committer/author grouping code and add a user-facing option that binds
them together. In particular:

  - the main option is now "--group", to make it clear
    that the various group types are mutually exclusive. The
    "--committer" option is an alias for "--group=committer".

  - we keep an enum rather than a binary flag, to prepare
    for more values

  - we prefer switch statements to ternary assignment, since
    other group types will need more custom code

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>