dgit: simplify gitattrs setup in mktree_in_ud_here

dgit: simplify gitattrs setup in mktree_in_ud_here

We are going to want to break the meat of mktree_in_ud here out into
Dgit.pm.  Dgit.pm doesn't have access_cfg, is_gitattrs_setup, and so
on.

On the other hand, we don't need any of that here: this is a fresh
tree of our own (so there is no existing .git/info/attributes to
combine with our new settings), and we ignore the config already.

So overall, little functional change: the .git/info/attributes in our
private working areas are now slightly simpler: they don't indirect
via git-defuse-attrs.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>