fix regexp pattern of diff

tool/format-release

@@ -235,11 +235,11 @@ eom
     diff.each_with_index do |line, index|
       case index
       when 0
-        line.sub!(/\A--- (.*)\t(\d+-\d+-\d+ [0-9:.]+ [\-+]\d+)\Z/) do
+        line.sub!(/\A--- (.*)\t(\d+-\d+-\d+ [0-9:.]+(?: [\-+]\d+)?)\Z/) do
           "--- a/#{filename}\t#{$2}"
         end
       when 1
-        line.sub!(/\A\+\+\+ (.*)\t(\d+-\d+-\d+ [0-9:.]+ [\-+]\d+)\Z/) do
+        line.sub!(/\A\+\+\+ (.*)\t(\d+-\d+-\d+ [0-9:.]+(?: [\-+]\d+)?)\Z/) do
           "+++ b/#{filename}\t#{$2}"
         end
       end