Before this patch, "reporelpath()" uses "rstrip(os.sep)" to trim
"os.sep" at the end of "parent.root" path.
But it doesn't work correctly with some problematic encodings on
Windows, because some multi-byte characters in such encodings contain
'\\' (0x5c) as the tail byte of them.
In such cases, "reporelpath()" leaves unexpected '\\' at the beginning
of the path returned to callers.
"lcalrepository.root" seems not to have tail "os.sep", because it is
always normalized by "os.path.realpath()" in "vfs.__init__()", but in
fact it has tail "os.sep", if it is a root (of the drive): path
normalization trims tail "os.sep" off "/foo/bar/", but doesn't trim
one off "/".
So, just avoiding "rstrip(os.sep)" in "reporelpath()" causes
regression around issue3033 fixed by e3dfde137fa5.
This patch introduces "pathutil.normasprefix" to normalize specified
path in the specific way for problematic encodings without regression
around issue3033.
