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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def find_submodule_git_dir(d):
 """Search for a submodule repo."""

    if is_git_dir(d):
        return d

    try:
        with open(d) as fp:
            content = fp.read().rstrip()
    except (IOError, OSError):
        # it's probably not a file
        pass
    else:
        if content.startswith('gitdir: '):
            path = content[8:]

            if Git.is_cygwin():
                ## Cygwin creates submodules prefixed with `/cygdrive/...` suffixes.
                path = decygpath(path)
            if not osp.isabs(path):
                path = osp.join(osp.dirname(d), path)
            return find_submodule_git_dir(path)
    # end handle exception
    return None