apply chdir monitor to all chdirs

Now that MRI and Rubinius also use the threaded installation plan, we
need to make sure that all installation threads will stay synchronized
and not cross-access files.
Fixed an issue with building git gems in parallel

We need to ensure that the dir remains the same
through out the installation of these gems,
this includes Path Installer as well as rubygems
build.
||= is not thread safe