Check another error condition in git-mv

When moving multiple files at once, it can happen that
files get the same target name, like in

git-mv a/foo b/foo destdir

Both a/foo and b/foo target destdir/foo.

Signed-off-by: Josef Weidendorfer <Josef.Weidendorfer@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>