.arcconfig

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

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