Ah, so if I were to manually craft a commit in a text editor in the format:

    tree sha1
    parent sha1 of parent I want to attach it to
    author some string
    committer some string

    The commit message

I could add this to the git object store manually under the same sha1 file and a client could just fetch it? Would the client try to fetch the faked objects when it already has the real objects in its copy of the object store?

That is, would it think it has the commit because the sha1 hasn't changed, but the tree sha1 has been updated and it would presumably refer to blobs that the client doesn't already have and try to fetch them. Or would it not proceed because it already has the commit?