Make RUBY_RELEASE_DATE full on development

I cannot tell the order just by commit hashes.

tool/file2lastrev.rb

@@ -11,6 +11,8 @@ require File.expand_path('../vcs', __FILE__)
 
 Program = $0
 
+TIMEZONE_FOR_RELEASE = "+09:00" # in Japan Standard Time traditionally
+
 @output = nil
 def self.output=(output)
   if @output and @output != output
@@ -74,6 +76,15 @@ vcs = nil
         if title
           "#define RUBY_LAST_COMMIT_TITLE #{title.dump}"
         end,
+        if modified
+          modified.getlocal(TIMEZONE_FOR_RELEASE).
+            strftime(<<TIME)
+#if defined(RUBY_PATCHLEVEL) && (RUBY_PATCHLEVEL == -1)
+#undef RUBY_RELEASE_DATE
+#define RUBY_RELEASE_DATE "%FT%T%:z"
+#endif
+TIME
+        end,
       ].compact
     }
   when :doxygen