It's both unexpected and undesirable for `brew bundle (exec|env|sh)` to
filter the environment and makes these tools less useful.
Not filtering the environment, though, causes issues with the
`brew bundle sh` shell. Fix this up and, while we're here, also improve
the formatting for both `zsh` and `bash` (the default) to use nicer and
more consistent prompts and colours.
To simplify this, consolidate some logic in a new
`Utils::Shell.shell_with_prompt` method and add tests for it and a
similar notice for `brew bundle sh`.
Finally, avoid printing out the notice when `HOMEBREW_NO_ENV_HINTS` is
set.
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