Fix bug #8098

Fix bug #8098

The Windows shell cannot have a UNC path as the current directory.
However, using pushd to change to the same directory automatically
maps the UNC path to a drive letter and thus it works.

Note that when calling cmd.exe from a UNC path there are still
warnings about the UNC path, but the command is then successfully
executed because the current directory is set to something digestible
by the shell.