rust-repo: make `Send` by not storing functions in `LazyCell` We (Google) want to use `Repo` in a context where we can store it in `Mutex<Repo>`. However, that currently doesn't work because it's not `Send` because the `LazyCell` initialization functions are not `Send`. It's easy to fix that by passing them to the `get_or_init()` and `get_mut_or_init()` functions. We'll probably also want `Repo` to be `Send` (and even `Sync`) in core later, so this seems like a step in the right direction. Differential Revision: https://phab.mercurial-scm.org/D12582