Pass -- to diff-tree for branch/filename disambiguation

Pass -- to diff-tree for branch/filename disambiguation

Script to show the need of this patch:

	mkdir ty;
	cd ty;
	git init-db;
	touch a;
	git add a;
	git commit -m "...";
	tg create bla;
	git commit -m "...";
	mkdir bla;
	touch bla/blub;
	git add bla/blub;
	git commit -m "...";
	tg export --quilt tt;

Reported-by: "Bernhard R. Link" <brlink@debian.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>