icasefs: use case preserved root for 'util.fspath()' invocation (issue3302) path to repo root may contains case sensitive part, even though repo is located in case insensitive filesystem: e.g. repo in FAT32 device mounted on Unix. so, case normalized root causes failure of stat(2). this patch uses case preserved root for 'util.fspath()' invocation to avoid this problem. case preserved root for 'util.fspath()' may decrease efficiency of fspath cache, but 'util.fspath()' is currently called only from dirstate, so this fix has less impact.