X-Debbugs-Cc: monnier@iro.umontreal.ca

Starting from "emacs -Q":

   M-x shell RET
   tar --directory=<TAB>
   ;; Notice that completions only show directories.

   M-x eshell RET
   tar --directory=<TAB>
   ;; Completions include files *and* directories(!)

After quite a bit of digging, I figured out the issue: Eshell was 
treating "--directory=" as the beginning of a local variable assignment 
(like when you run "CC=gcc make"). The function 
'eshell-complete-variable-assignment' is over-aggressive in identifying 
local variable assignments, and so it picked this up as a false positive.

Attached is a fix for this, plus a regression test.