When contributing patches to Git, developers often need to compare in what form those patches made it upstream (e.g. to pick up fixes made by the Git maintainer in preparation for a new iteration). In such a case, a --left-only (or --right-only) option would come in real handy, to allow git range-diff --left-only <branch>..upstream/pu to skip commits that are upstream only (as there will be a metric ton of them).