procutil: add a option to not fully detach background process
When writing test, it is useful to make code pretending to spawn a fully
detached process while still actually waiting for it to finish. We add the
option to `procutil.runbgcommand`, to do so. We will use this in remote-filelog
tests.
Differential Revision: https://phab.mercurial-scm.org/D7584
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