dirstate: use tuple interface to fix leak in pack_dirstate()
Spotted by ASAN.
Unlike PyTuple_GET_ITEM(), PySequence_ITEM() returns a new reference. This
bug could be fixed by inserting Py_CLEAR() and Py_XDECREF() appropriately,
but I think requiring a tuple object is simpler and less error-prone.
The cext version is jumped to 10 since 6..9 are used in the default branch.
We'll need to bump it again at merge.
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