gitattributes not read for diff-tree anymore in 2.15?

Hi,

I've bisected a failure in our test suite to this commit:

    commit 557a5998df19faf8641acfc5b6b1c3c2ba64dca9 (HEAD, refs/bisect/bad)
    Author: Brandon Williams <bmwill@google.com>
    Date:   Thu Aug 3 11:20:00 2017 -0700

        submodule: remove gitmodules_config

        Now that the submodule-config subsystem can lazily read the gitmodules
        file we no longer need to explicitly pre-read the gitmodules by calling
        'gitmodules_config()' so let's remove it.

        Signed-off-by: Brandon Williams <bmwill@google.com>
        Signed-off-by: Junio C Hamano <gitster@pobox.com>

Which is tags/v2.15.0-rc0~120^2.

Our test suite is in a Rust project here:

    https://gitlab.kitware.com/utils/rust-git-checks

and the failing test(s) can be run using:

    cargo test whitespace_all_ignored

The test checks that when `.gitattributes` says that whitespace errors should be ignored, they aren't reported as errors. My guess is that not reading the gitmodules configuration also skips reading attributes files. Is this reasoning correct?

Thanks,

--Ben