Redmine 6523: Avoid read/write access to Setuid storage

This clears the list, forcing it to be reloaded (if needed) next time this function is called.
Is it possible this function is called repeatedly, during a single call to VerifyFilePromise() ?
It seems to get called under VerifyFileLeaf(), which is called by DepthSearch(), which sounds like a traversal that could make many calls, but I'm not familiar with this code !
Previously, each call to VerifyFilePromise() loaded and unloaded this exactly once; if we're now potentially reloading it many times during one call, there's a performance downside.