bundle2: mark the bundle2 part as advisory (issue5872)
It blocks old clients to read bundle including this part.
Differential Revision: https://phab.mercurial-scm.org/D3481
wrap 0: [0, 'orig']
wrap 1: [1, 0, 'orig']
wrap 2: [2, 1, 0, 'orig']
wrap 3: [3, 2, 1, 0, 'orig']
wrap 4: [4, 3, 2, 1, 0, 'orig']
wrap 0: [0, 4, 3, 2, 1, 0, 'orig']
unwrap 3: 3: [0, 4, 2, 1, 0, 'orig']
unwrap -: 0: [4, 2, 1, 0, 'orig']
unwrap 0: 0: [4, 2, 1, 'orig']
unwrap 4: 4: [2, 1, 'orig']
unwrap 0: -: ValueError
unwrap 2: 2: [1, 'orig']
unwrap 1: 1: ['orig']
unwrap -: -: IndexError
context manager ['orig']
context manager [1, 'orig']
context manager [0, 1, 'orig']
context manager [2, 0, 1, 'orig']
context manager [2, 1, 'orig']
context manager [2, 'orig']
wrap callable object [0, 'orig']