Fixes for using mercurial-ruby on Windows

[Tak]

I hit two stumbling blocks when trying to use mercurial-ruby on windows:

• popen4 ultimately calls fork, which is unsupported
• shell tries to wrap command arguments in single quotes, which the windows shell hates

I've verified that neither change regresses existing tests (open3 codepath passes everything, double-quote codepath passes everything except tests to ensure that we're using single quotes to wrap)

[cignoir]

:+1: This fixes #6.

[Tak]

So...is this repo abandoned?