[FIXED JENKINS-18283] Wrong URL in diff (and file view)

When ViewVC is not install at root of server (http://www.mysvnserver.domain.com/) , for example: 
http://www.mysvnserver.domain.com/viewvc
or
http://www.mysvnserver.domain.com/bin/viewvc.cgi

Then the diff link and view file link is not correct because it will append the SVN path to the bare URL (i.e. lost the "context" path):
http://www.mysvnserver.domain.com/SVN/path/here/as/checked/out/myfile.txt

The patch is my attempt to fix those URLs.