Every remote could have its own list of "revs that were on branch". Gitlab, or whatever is your server of choice, would then automatically record this on merges

Every remote could have its own list of "revs that were on branch". Gitlab, or whatever is your server of choice, would then automatically record this on merges. Most code review tools already has this information stored in their databases outside of git. It would need to be transferred on the side and not in the commits themselves to not rewrite commits in case of merges from other remote. Similar to tags, but instead of only pointing to single commit it could be a list, let's call them labels. Git log would then add option like --labeled-by=origin/merged-on-master