bug#2989: implement bash's ignoredups

[jidanni]

Dan Nicolaescu <dann@ics.uci.edu> writes:
> *** New user option `history-delete-duplicates'.
Well, OK, but see, there they still are at the <escape> p's below.
$ cd /tmp
$ echo a:\;date>Makefile
$ emacs -Q -eval '(setq history-delete-duplicates t)' Makefile
<escape> x c o m p i l e <return> <return> <escape> x <up>
<return> <return> <escape> x <up> <return> <return>
C-x <escape> <escape> <escape> p <escape> p <escape> p
emacs-version "22.3.1" (Yes, I still use <escape>).