rust-files: also return filenode and flags when listing a revision's files This is going to be useful when implementing parts of `update` and makes it so we don't have to fetch the manifest and each entry twice.

rust-manifest: encode flags as `Option<NonZeroU8>` This makes the compiler use the niche optimization for all flags: since 0 is not a valid representation of any flags, we can use 0 as a replacement for `None`, which reduces memory footprint and could yield a little performance improvement over many iterations.

rust-repo: add a method to set the current parents This will be useful when we start writing an `hg update` fastpath.

rust-repo: add a method to get a `Node` from a `Revision` to the `Repo` This is going to be more and more useful as we start writing higher-level code

rust-utils: move the `filesystem_now` function to a util This is going to be useful for an upcoming `hg update` fastpath.

rust-dirstate: actually remember the identity This was an oversight that likely had no influence on anything since we have only been writing the dirstate for status. We will start writing the dirstate for more operations now, so we'll need this fixed.