Shifted config path resolution along 1 char to avoid '\\' path issue

When serializing some items from deeper in the tree, we kept coming up against a 'Path is not under the root' error.  This was being caused by Sitecore having to use GetShortPath() as the path to the item was too long and 'ResolveConfigurationPath' was returning a path with '\\\\' for web root paths.

While most file-related methods can cope with a path that contains '\\\\', eg 'C:\github\sitecore\\project1', GetShortPath() doesn't.

You can recreate this by having an item in your tree with a path of longer than 150 characters (assuming a SerializationFolderPathMaxLength of 90), and setting your serializationProvider rootPath to a web-root-relative path.