tests/test-gendoc-sv.t

procutil: allow to specify arbitrary  stdin bytes to runbgcommand

For automatic clonebundles generation I need to pass arbitrary large amount of
data to the process (eg: common nodes, target nodes).

I am updating the `runbgcommand` to allow for this. Previously not stdin input
was possible, now, one can provide raw bytes and they will be feed to the
command through an unnamed temporary files.

Differential Revision: https://phab.mercurial-scm.org/D9212

#require docutils gettext

  $ $TESTDIR/check-gendoc sv
  checking for parse errors