dirs._addpath: reinstate use of Py_CLEAR I changed this to an explicit Py_DECREF + set to null in 6f0e6fa9fdd7. This was a silly misunderstanding on my part -- for some reason I thought Py_CLEAR set its argument to null only if its refcount reached 0. Turns out that's not actually the case -- Py_CLEAR is just Py_DECREF + set to null with some additional precautions around destructors that aren't relevant here. The real bug that 6f0e6fa9fdd7 fixed was the fact that we were mutating the string after setting it in the Python dictionary.

exchange: introduce a '_canusebundle2' function This function refactors the logic that decides to use 'bundle2' during an exchange (pull/push). This will help being consistent while transitioning from the experimental protocol to the final frozen version. I do not expect this function to survive on the long run when using 'bundle2' will become a simple capability check. This is also necessary to allow HG2Y support in an extension to ease transition of companies using the experimental protocol in production (yeah...). Such extension will be able to wrap this function to use the experimental protocol in some case.