icasefs: use util.normcase() instead of lower() or os.path.normcase in fspath this also avoids lower()-ing on each path components by reuse the path normcase()-ed at beginning of function.

dirstate: prevent useless util.fspath() invocation for '.' at first of dirstate.walk() on case insensitive filesystem, normalization of '.' causes util.fspath() invocation, but '.' is not cached in it. this invocation is not only useless, but also harmful: initial "hg tag" causes creation of ".hgtags" file after dirstate.walk(), and looking up ".hgtags" in cache will fail, because directory contents of root is already cached at util.fspath() invocation for '.'.