git-debrebase: walk: Reintroduce the local $read_tree_upstream sub

git-debrebase: walk: Reintroduce the local $read_tree_upstream sub

Now that we have the $tree_with_debian parameter to the global
read_tree_upstream, we can use it to avoid the git-write-tree call
and pass $build as the $tree_with_debian.

This thus partially reverts
  dfc092e0ec3d932e51a071000519477d0e33525c
  git-debrebase: Make read_tree_upstream global (nfc)

The combined effect is now that $read_tree_upstream does exactly the
git commands it did before.

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