This commit triggers precmd and chpwd hook functions iff we changed directory.

This has the same behavior as zsh's hook function execution, which tries to run
the functions in the order specified and silently ignores any function that
does not exist.

See http://zsh.sourceforge.net/Doc/Release/Functions.html#Hook-Functions

Also moved duplicate nopushdminus logic to the `switch-to-dir` function.