remote-hg: fix for older versions of python

As Amit Bakshi reported, older versions of python (< 2.7) don't have
subprocess.check_output, so let's use subprocess.Popen directly as
suggested.

Suggested-by: Amit Bakshi <ambakshi@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>