procutil: allow callers of runbgcommand to assume the process starts
Experimentally starting the subprocess can take as much as 40ms, and
for some of our use cases that's frivolous: we know the binary will
start, and if it doesn't we'd only ever ignore it and continue
anyway. This lets those use cases be faster.
Differential Revision: https://phab.mercurial-scm.org/D6537