diff: Modify `TestDiffBlobs` to acknowledge `SetAttrTreeConfig`

The `TestDiffBlobs` tests assume that the 'attr.tree' config is set to 'HEAD'. While this is true in Git versions < 2.46.0, this patch was reverted in master and will land in 2.46.0. This means it will no longer be default. We have thus created the `SetAttrTreeConfig` to capture this behavior. But this flag value is not used in `TestDiffBlobs`, which sets some attributes. Modify the test to acknowledge this flag and change the expected response correspondingly.