Fix updating repository with externals using per job credentials

There's a problem when using per job credentials with the repository having external links. In our particular case we have a LOT of external links to different locations of the same repository.

Let's assume that we have:
 - svn://root/ - root repository folder
 - svn://root/assembly/ - folder with many external links
 - svn://root/package/a/trunk ... svn://root/package/z/trunk - folders pointed by external links

Though a project is configured to point to svn://root/assembly/, hudson tries to authentificate to individual svn://root/package/.../trunk when commit log is built. Unfortunately, per job credentials are mapped by url and are returned for svn://root/assembly/ solely. No problem appears when global credentials are used, since they are mapped by svn realm.

I suggest to automatically check each shorter url when looking for per job credentials:
 - svn://root/package/a/trunk
 - svn://root/package/a/
 - svn://root/package/
 - svn://root/

In other words, credentials given for some URI are also to be applied to any sub-URI.

This will perfectly solve that particular issue we talk about provided that credentials for svn://root/ are given.