"cd" redefinition should only apply to interactive shells

[sarowe]

"cd" is overridden in bash-idh to trim duplicate entries from Bash's directory stack.

Non-interactive shells (scripts) exhibit bad behavior from side effects of the cd redefinition. (I don't remember the details, just that "cd" in some scripts does the wrong thing.)

It should be possible for the redefinition to only apply to interactive shells.