Add an attribute accessor to CanonicalTreeParser and use it in Treewalk

Add an attribute accessor to CanonicalTreeParser and use it in Treewalk

When checking out a branch we need to access the attributes stored
in the tree to be checked out. E.g. directly after a clone we checkout
the remote HEAD. In this case index and workingtree are still empty.
So we have to search the tree to be checked out for attributes.

Change-Id: I6d96f5d095ed2e3c259d4b12124e404f5215bd9f