case-insensitive partial-completion-mode issue

[rahul]

(setq partial-completion-mode t)
(setq read-file-name-completion-ignore-case t)

C-x C-f fi <tab> completes to FILE.TXT
C-x C-f DI/FI <tab> completes to DIRECTORY/FILE.TXT

but, C-x C-f di/fi <tab> won't complete to DIRECTORY/FILE.TXT

i found a similar bug in emacs-devel:
http://lists.gnu.org/archive/html/emacs-devel/2006-11/msg00706.html

and a recent patch for it:
http://lists.gnu.org/archive/html/emacs-devel/2007-10/msg00053.html

but it didn't fix this problem (tested against current cvs).