Update win32mbcs extension
* Code cleanup by Matt.
* Fix the issue with case-insensitive fs support
by wrapping also util.fspath() and util.checkcase()
* Abort program when path conversion is failed.
# should print nothing
# should fail
abort: path 'b/x' is inside repo 'b'
abort: path 'b/x' is inside repo 'b'
# should fail
abort: path 'b/x' is inside repo 'b'
# should arguably print nothing
# should fail
abort: path 'b/a' is inside repo 'b'