The one place a child process is started (mtools:W873), extracted from mikemol.treeio.proc so a caller that only needs to start a child does not inherit treeio's native git dependency. A library: no console script, standard library only.

Behaviour: the argv is a sequence, never a shell string; a non-zero status is a value, not a raise; the working directory and the whole environment are the ones passed; a child that outlasts its timeout raises subprocess.TimeoutExpired, and with no timeout nothing is bounded.