commit: Various fixes for `CheckObjectsExist()`
As part of #3751 (closed) I was trying to implement a method in Rails that wraps our new CheckObjectsExist()
RPC. I couldn't get it to work though because I never got any results back, and it took me quite a while to figure out that this is in fact an issue on our own side because we ignored revisions sent as part of the initial request. This MR fixes this and some other bugs in the RPC to make it easier to use.