Documentation/config: don't list date formats

This list is already incomplete (missing "raw") and we're about to add
new formats.  Since this option sets a default for git-log's --date
option, just refer to git-log(1).

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1781,9 +1781,7 @@ log.abbrevCommit::
 log.date::
 	Set the default date-time mode for the 'log' command.
 	Setting a value for log.date is similar to using 'git log''s
-	`--date` option.  Possible values are `relative`, `local`,
-	`default`, `iso`, `rfc`, and `short`; see linkgit:git-log[1]
-	for details.
+	`--date` option.  See linkgit:git-log[1] for details.
 
 log.decorate::
 	Print out the ref names of any commits that are shown by the log