dirstate-item: introduce low level C function Since a lot of logic is about to change, we need to isolate it into bits that are easy reusable in the rest of the C code. Especially we need to split the part that do the pure logic from the part that do the Python wrapping. Differential Revision: https://phab.mercurial-scm.org/D11386

dirstate-item: factor some code in the C implementation We have both `dirstate_item_from_v1_meth`, a class method, and `dirstate_item_from_v1_data`, a function taking low level C argument. Lets implement the former with the later. Differential Revision: https://phab.mercurial-scm.org/D11385

dirstate-item: `dirstate_item_from_v1_data` replaces make_dirstate_item For different format, we will need multiple capsule anyways. Differential Revision: https://phab.mercurial-scm.org/D11384