DeltaWalker is a non-free tool popular among some users. Add a
plug-in to support it from difftool and mergetool.
Note that the $(pwd)/ in front of $MERGED should not be necessary.
However without it, DeltaWalker crashes with a JRE exception.
Signed-off-by: Tim Henigan <tim.henigan@gmail.com>
Helped-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
