def get_header_hgarg(environ):
    """Decode the special Mercurial encoding of big requests over multiple headers.
    >>> get_header_hgarg({})
    ''
    >>> get_header_hgarg({'HTTP_X_HGARG_0': ' ', 'HTTP_X_HGARG_1': 'a','HTTP_X_HGARG_2': '','HTTP_X_HGARG_3': 'b+c %20'})
    'ab+c %20'
    """
    chunks = []
    i = 1
    while True:
        v = environ.get('HTTP_X_HGARG_%d' % i)
        if v is None:
            break
        chunks.append(v)
        i += 1
    return ''.join(chunks)
