bug#6764: 24.0.50; directory tracking functions (comint.el) non-functional on remote files

Steps to reproduce:
1. Open a tramp url like "/ssh:user <at> HOSTNAME:/"
2. M-x shell
3. cd /absolute/path
4. Check `default-directory' or try comints directory completion/tracking
Patch enclosed:

diff --git a/lisp/comint.el b/lisp/comint.el
index 071537f..128965f 100644
--- a/lisp/comint.el
+++ b/lisp/comint.el
 <at>  <at>  -674,6 +674,9  <at>  <at>  Entry to this mode runs the hooks on `comint-mode-hook'."
   (make-local-variable 'comint-process-echoes)
   (make-local-variable 'comint-file-name-chars)
   (make-local-variable 'comint-file-name-quote-list)
+  ;; dir tracking on remote files
+  (set (make-local-variable 'comint-file-name-prefix)
+       (or (file-remote-p default-directory) ""))
   (make-local-variable 'comint-accum-marker)
   (setq comint-accum-marker (make-marker))
   (make-local-variable 'font-lock-defaults)

If Emacs crashed, and you have the Emacs process in the gdb debugger,
please include the output from the following gdb commands:
    `bt full' and `xbacktrace'.
For information about debugging Emacs, please read the file
/usr/share/emacs/24.0.50/etc/DEBUG.

In GNU Emacs 24.0.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 2.20.1)
 of 2010-07-30 on bitzer.hoetzel.info