revset: add a utility for obtaining the source of a given rev graft, transplant and rebase all embed a different type of source marker in extra, and each with a different name. The current implementation of each is such that there will never be more than one of these markers on a node. Note that the rebase marker can only be resolved if the source is still present, which excludes the typical rebase usage (without --keep) from consideration (unless the resulting bundle in strip-backup is overlayed). There probably isn't any reason to use rebase --keep as a substitute for transplant or graft at this point, but maybe there was at one point and there are even a few rebases in the hg repo, so it may be of historical interest.