attr.tree::
	A reference to a tree in the repository from which to read attributes,
	instead of the `.gitattributes` file in the working tree. In a bare
	repository, this defaults to `HEAD:.gitattributes`. If the value does
	not resolve to a valid tree object, an empty tree is used instead.
	When the `GIT_ATTR_SOURCE` environment variable or `--attr-source`
	command line option are used, this configuration variable has no effect.
