populate: exit with the status of the command that failed

The shell implementation runs under set -e, so it exits with the status
of the command that failed, e.g. 255 if ssh can't reach the target,
which wrappers retrying on connection failures rely on, or 128 if git
fetch fails.  The program does the same, instead of exiting with 1, and
with 128 plus the signal for a command a signal ended.

A phase's expect.status is the exit status it expects populate to fail
with; git-status checks git fetch's.

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