Merge branch 'jc/attr-tree-config'

Merge branch 'jc/attr-tree-config'

The attribute subsystem learned to honor `attr.tree` configuration
that specifies which tree to read the .gitattributes files from.

* jc/attr-tree-config:
  attr: add attr.tree for setting the treeish to read attributes from
  attr: read attributes from HEAD when bare repo