bug#22679: 25.0.91; ibuffer-do-shell-command-pipe truncate output

[Glenn Morris]

This seems like it would be a lot simpler if shell-command(-on-region)
did not unconditionally erase its output buffer. Although that behaviour
is long-standing, it seems unfriendly. It would be easier for callers
that wanted that to erase their own output buffers. It's less simple for
callers that want to preserve existing output to do so with the current
system.