Computing the number of patches in linux-next tree

I tried to produce a graph of the pending avalanche of patches sitting in the linux-next tree and came up with this:

git tag | grep next- | sort | while read tag
do
        c=$(git-rev-list --no-merges $tag ^linus | wc -l)

        echo ${tag##next-} $c
done

Where "linus" is a branch that tracks Linus' tree. But this gets the wrong count for old tags because maintainers who use the topic-branch method have exactly the same commits in linux-next as eventually end up in Linus ... so once Linus pulls these trees, the historical counts change.

What I really need at this point is the commit from Linus tree that Stephen uses as the basis for each next-* tag. I.e. change the loop body to

       base=`git-where-did-this-tag-branch-from-linus $tag`
       c=$(git-rev-list --no-merges $tag ^$base | wc -l)

Is this the right way to do this? What does the "git-where-did-this-tag-branch-from-linus" command look like?

-Tony