redmine-backporter.rb: suppress a warning [ci skip]

* tool/redmine-backporter.rb (StringScanner.readline): use `true`
  to suppress a "literal in condition" warning.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63458 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/redmine-backporter.rb

@@ -215,7 +215,7 @@ class << Readline
     console.print prompt
     console.flush
     line = ''
-    while 1
+    while true
       case c = console.getch
       when "\r", "\n"
         puts