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