rust-index: handle `MixedIndex` in `pyindex_to_graph`
On the long run we will want to implement the Graph trait directly in Rust, but
for now we take the path with the least amount of change to focus on the coming
persistent NodeMap code.
We test this new code through with the lazy ancestors code.
Differential Revision: https://phab.mercurial-scm.org/D7657
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