Add general funciton to load operations

As an analogous to versioncontrol_repository_load_multiple() it would be handy to have a general loader for operations.

We now can do a $repository->loadCommits(), but that works from an instantiated VersioncontrolRepository object, which means it works for commits on that repository IIRC, so this loader should be repository agnostic.

I en d up writing some initial code for this on #1024958: Commitlog should provide RSS feeds of each view, but since I am discarding it from the patch there I am opening this just in case we want to add this feature to the API.