completion: add more parameter value completion

This adds value completion for a couple more paramters. To make it
easier to maintain these hard coded lists, add a comment at the original
list/code to remind people to update git-completion.bash too.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

date.c

@@ -921,6 +921,10 @@ static enum date_mode_type parse_date_type(const char *format, const char **end)
 		return DATE_UNIX;
 	if (skip_prefix(format, "format", end))
 		return DATE_STRFTIME;
+	/*
+	 * Please update $__git_log_date_formats in
+	 * git-completion.bash when you add new formats.
+	 */
 
 	die("unknown date format %s", format);
 }