git/Documentation/config/attr.txt

attr.tree::
	A reference to a tree in the repository from which to read attributes,
	instead of the `.gitattributes` file in the working tree. 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.