git-debrebase: Run gbp pq export only from (perhaps private) laundered branch

git-debrebase: Run gbp pq export only from (perhaps private) laundered branch

When the branch is stitched, gbp pq will descend into both sides of
the pseudomerge.  This will duplicate patches and be generally bad.

So instead, use walk to generate, privately, a laundered version of
HEAD, to use to generate the patches.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>