Merge branch 'jc/no-default-attr-tree-in-bare' into maint-2.45

Earlier we stopped using the tree of HEAD as the default source of
attributes in a bare repository, but failed to document it.  This
has been corrected.

* jc/no-default-attr-tree-in-bare:
  attr.tree: HEAD:.gitattributes is no longer the default in a bare repo