rust-dirstatemap: use `get_node_mut` instead or `get_or_insert_node`...
rust-dirstatemap: use `get_node_mut` instead or `get_or_insert_node`
This (along with the docstring), makes it more obvious that we're not expecting
to insert a node here. This is less prone to bugs in later refactorings.
Differential Revision:
https://phab.mercurial-scm.org/D12530
