git does have a concept of a merge commit, and most pull request tools have the option to force merge commits (as opposed to and in contrast to fast-forward merges or squash/rebase merges). If merge commits are forced you have an object representing completed PRs in the DAG. You can use tools like git log --first-parent to browse the "PR list" without needing to get into weeds of the individual commits "inside" the PRs at first.
