git-archive and unwanted .gitattributes

Hi,

Currently attr.c will read .gitattributes on disk no matter there is a real worktree or not. This can lead to strange behavior. For example when I do

mkdir a && cd a git init echo '$Format:%s$' > a git add a && git commit -m initial cd .. echo 'a export-subst' > .gitattributes # let's assume this is an accident. git --git-dir=a/.git archive --format=tar HEAD|tar xO a

I expect it to show '$Format:%s$', not "initial". git-archive should not bother reading that .gitattributes. I thought an is_inside_work_tree() check would be enough. Unfortunately, setting --git-dir will automatically set worktree too. Any ideas?