shortlog: accept `--date`-related options

shortlog: accept `--date`-related options

Prepare for a future patch which will introduce arbitrary pretty formats
via the `--group` argument.

To allow additional customizability (for example, to support something
like `git shortlog -s --group='%aD' --date='format:%Y-%m' ...` (which
groups commits by the datestring 'YYYY-mm' according to author date), we
must store off the `--date` parsed from calling `parse_revision_opt()`.

Note that this also affects custom output `--format` strings in `git
shortlog`. Though this is a behavior change, this is arguably fixing a
long-standing bug (ie., that `--format` strings are not affected by
`--date` specifiers as they should be).

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