refs/replace advice

I've got two near-identical git repos, both imported from gigantic upstream p4 repos. They started at slightly different times so have different commit SHA1s, even though the tree contents are the same. I can't filter-branch either of them; too many users already.

I'm trying to use "git replace" to avoid cloning the entire set of duplicate commits across a slow inter-site link. Like this:

    ...---A----B----C   site1/top
                     \
                      D---E---F  site1/proj

    ...---A'---B'---C'  site2/top

It is true that "git diff C C'" is empty:  they are identical.

This set of commands, run from site2, clones most of the repo locally (up to C'), then grabs the few changes D..F from the faraway site1:

    git clone /path/to/site2.git repo
    cd repo
    git remote add -f site1 /path/to/faraway/site1.git

But it causes an entire fetch of all commits because C != C'. I'd prefer it just to fetch D, E and F. So I try:

    git refs replace A' A

but it still fetches everything. I toyed with grafting site1's A on top of the parent of our local A':

    echo A A'^ > .git/info/grafts

no luck.

I thought maybe I could "git fetch --depth=N" where N would cover the range A'..site2/top, then replace. But testing with "git fetch --depth=3" still wants to fetch 100k objects.

Any ideas?

                -- Pete