mergetool--lib: specialize diff options for emerge and ecmerge

mergetool--lib: specialize diff options for emerge and ecmerge

The ecmerge documentation mentions the following form:

	ecmerge --mode=diff2 $1 $2

Since git-difftool is about diffing, we should use that instead
of --mode=merge2.  Likewise, this drops the $MERGED argument to
emerge, as discussed on the git list ($gmane/117930).

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>