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
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