Yes, a branch together with information about the intended point of divergence is a stack of commits. (Most of the time, the point of divergence is arrived at implicitly by specifying the intended mother branch, and git figures out the last common ancestor, ie the merge-base.) Let's call the branch-plus-point-of-divergence a 'PR branch'.
Though your 'stack of commits' is more like a 'stack of diffs' (at least in the Git sense of commit-is-a-snapshot-of-the-whole-repo.)
And actually, for better or worse, a PR branch is more expressive than a 'stack of commits', exactly for the reason you suggest: a PR branch can also contain merge commits, it doesn't have to be a linear chain.
Basically in terms of git speak branches are mutable pointers to immutable commits. (They are basically the only mutable thing in git.)
What you'd need to do to support 'stacked diffs' or 'stacks of commits' is to keep a history of what you mutable pointers were doing. It's a 'meta-git'.
Once you have that, you could build all the convenient operations on top of that abstraction.
Btw, if memory serves right, git has some plugins (or whatever these things are called, plugins or so?) for implementing the stack of commits concept.
I don't know whether they go the 'meta-git' route that I sketched, or whether they do something less abstract.
