We very often expect success, and we sometimes want to get the stdout,
too. Let's add a convenience function for that. It saves a lot of
lines of code.
As pointed out by @arxanas in #88, the message saying something like
"At least 'bin/.DS_Store' was added back ..." is confusing especially
when the command you ran was just `jj untrack bin/.DS_Store`. Let's
clarify the message by saying exactly how many more files there are,
and specialize the message for when there is only one file. Also
update the message to say "would be added back" instead of "was added
back" since we don't actually change anything if some files would be
added back (since 4b91ad408c).
Should we even list all the files? I'm concerned that such a list
could be very long. On the other hand, it can also be annoying to have
to run `jj untrack some/dir/` and only be told about single file to
add to the ignore patterns every time.
