rust-index: add a function to convert PyObject index for hg-core Function in hg-core need something implementing the `Graph` trait. Right now, the `hg-cpython` entry points directly turn the PyObject passed as argument into a `cindex::Index`. However, if we start having the option to use an Index in Rust, we need to dispatch between the different possible PyObject we could receive. So move the "duplicate" call into a unified function. When time come. It will be easy to update the logic of all interface when the time come. Differential Revision: https://phab.mercurial-scm.org/D7653