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>