rust-revlog: move check for nodemap requirement to caller It's good for both making `Revlog` testable and reusable to have it not depend on the higher-level `Repo` type. This patch is one step in towards that. Additionally, this change in particular gives the callers more control over when to use a nodemap. Differential Revision: https://phab.mercurial-scm.org/D12546

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