Split run_command into two halves (start/finish)

Split run_command into two halves (start/finish)

If the calling process wants to send data to stdin of a
child process it will need to arrange for a pipe and get
the child process running, feed data to it, then wait
for the child process to finish.  So we split the run
function into two halves, allowing callers to first
start the child then later finish it.

Signed-off-by: Shawn O. Pearce <redacted>
Signed-off-by: Junio C Hamano <redacted>