Given git's support for merge trees, and arbitrary workflows, this already exists, it's just hard to access. With git, you can create a commit history that looks like this:

    A
    |\
    | B
    |/
    C

and git log --first-parent will show A and C, and hide B, even though B's code is contained in A. We used this all the time at Ksplice, where we had some custom tooling that made it easy create those triangles and our build system would use the commit message on A while ignoring B in order to build updates.