For example, https://git-scm.com/docs/gitrepository-layout says:

    info
    Additional information about the repository is recorded in this directory.
    This directory is ignored if $GIT_COMMON_DIR is set
    and "$GIT_COMMON_DIR/info" will be used instead.

So when looking for `info/attributes`, we need to check the commondir first,
or fallback to "our" `info/attributes`.