exp/cmd/upbox: print shell command before execution
exp/cmd/upbox: print shell command before execution

This allows users to copy the command-line to run an additional shell
session in another terminal.

Change-Id: I6c869c6313a2ddcae76d96ab1048fa8222cba63b
Reviewed-on: https://upspin-review.googlesource.com/8681
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin