Add --reverse to the git-rev-list usage string

git-rev-list accepts --reverse, as documented in
the manpage, but the usage string does not list it.

Signed-off-by: Kevin Ballard <kevin@sb.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-rev-list.c

@@ -33,6 +33,7 @@ static const char rev_list_usage[] =
 "  ordering output:\n"
 "    --topo-order\n"
 "    --date-order\n"
+"    --reverse\n"
 "  formatting output:\n"
 "    --parents\n"
 "    --objects | --objects-edge\n"