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>

shortlog.h

@@ -2,6 +2,7 @@
 #define SHORTLOG_H
 
 #include "string-list.h"
+#include "date.h"
 
 struct commit;
 
@@ -15,6 +16,7 @@ struct shortlog {
 	int in2;
 	int user_format;
 	int abbrev;
+	struct date_mode date_mode;
 
 	enum {
 		SHORTLOG_GROUP_AUTHOR = (1 << 0),