transplant: forbid transplant to nonempty repositories with no working directory. Partially undoes d3fb413667e5.

ui.isatty() accesses sys.stdin, causing mod_wsgi to throw IOError.

hg: allow hg.parseurl(url, None) In many places hg.parseurl is called with a url and "opts.get('rev')", suggesting the second, optional argument can be None. Because opts['rev'] usually defaults to [] this never happens in practice. However, extensions don't necessarily behave the same, but do copy this pattern. Also, include wider hg.parseurl tests, beyond a demonstration of the problem.