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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 commit_hash(dir='.'):
 """ Return commit hash for HEAD of checked out branch of the specified directory. """


    cmd = ['git', 'rev-parse', 'HEAD']

    try:
        with open(os.devnull, 'w') as devnull:
            revision_hash = subprocess.check_output(
                cmd,
                cwd=dir,
                stderr=devnull
            )
        if sys.version_info.major > 2:
            revision_hash = revision_hash.decode('ascii')

        return revision_hash.strip()

    except subprocess.CalledProcessError:
        return None