git can already do this as long as there isn't a conflict. Maybe pijul has better three way conflict resolution, but those can be risky, and avoiding those rare situations wouldn't offset the vast amount of tooling that got has.
Unless your VCS is handling CI for integrating changes on push, you really need to pull down the upstream changes first and test them combined with your code before blindly pushing.
Pijul has a theory of textual changes, but indeed doesn't care at all about what you write in your files: that's your problem!
A good example for this is code which grabs several locks and different functions have to do that in the same order, or a deadlock will result. A lot of interaction, even if changes might happen in completely different lines.
And I think that's generally true for complex software. Of course it is great if the compiler can prove that there are no data race conditions, but there will always be abstract invariants which have to be met by the changed code. In very complex code, it is essential to be able to do bisecting, and I think that works only if you have a defined linear order of changes in your artefact. Looking at the graphs of changes can only help to understand why some breakage happened, it cannot prevent it.
Darcs is older than git.
In my defense, 3-way merges are older :)
You have two repos with different history:
master -> patch a -> patch b
master -> patch b -> patch a
But the contents of the files are equal after applying both patches (in either order).
Git will consider these to be two different, Pijul thinks they're the same.
This is a simplified example. It only gets interesting when there are a lot of patches, some of which are commutative and some are not.
If the VCS knows about dependencies between patches intuitively, it could free me from having to explain it, which in the case of Git, requires following procedures that I'm unlikely to convince any of my coworkers to follow ("ohay, first, decide on the earliest point in the history from which this patch could make sense, rebase onto that....")
You and I, who are both working off of main, and who have both separate merged in a few patchsets that are relevant to our shared module of interest. We can merge and compare our branches, and the differences in terms of nursing patches without having to be rigorous about reconciling or histories.