Fixed a bug that can cause 404 in the form validation check. Contextual...

Fixed a bug that can cause 404 in the form validation check. Contextual ancestor portion shouldn't be cached.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@28217 71c3de6d-444a-0410-be80-ed276b4c234a