d-r-s- should sensibly print executed commands in emailed logs

d-r-s sends emails which are supposed to contain various helpful logs. I think these include the stdout/stderr from commands it runs. But it ought to also include the commands that were run.

At least, some parts of the commands: maybe we don't want to print out the whole ssh to builder and autopkgtest shell command. And, at least, some of the commands: maybe we only want to print "uninteresting" commands (eg git init, mkdir) if they fail.

Work items for this ticket:

• Make a test case or ad-hoc setup that runs d-r-s without stuff that will add debug options to d-r-s and dgit.
• Ensure (and maybe add a test) that at least the main part of of the "dgit fetch" and "git fetch" commands is printed.
• If we decide to have "uninteresting" commands which aren't routinely printed, check (and maybe add a test) that if some other command fails, the command is printed.
• Decide if the output ought to include all the ssh builder autopkgtest virt stuff, and implement that decision