Add new config variable $wgFileCacheDepth to set the depth of the subdirectory hierar...

Add new config variable $wgFileCacheDepth to set the depth of the subdirectory hierarchy used for the file cache. Default value is 2, which matches former behavior. Setting this to 0 makes the paths simpler, and can be used with clever Apache rewrite rules to serve cached files directly without calling MediaWiki or PHP at all.