Toggle directory tracking in this shell buffer (Shell Dirtrack mode).
With a prefix argument ARG, enable Shell Dirtrack mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil.
The `dirtrack' package provides an alternative implementation of
this feature; see the function `dirtrack-mode'.
(fn &optional ARG)
Non-nil if Shell-Dirtrack mode is enabled.
Use the command `shell-dirtrack-mode' to change this variable.