git-debrebase: DgitImportUnpatched: defer upstream diff check

git-debrebase: DgitImportUnpatched: defer upstream diff check

We can't do this during the initial walk, because what we ought to be
comparing is the previous breakwater with the new one.  So we have to
wait until the walk has proceeded down the overwritten branch, to find
the old breakwater.

This can be done by waiting until we are rebuilding the branch: at
that point, we have just built the previous breakwater, so we can
compare it to the dgit unpatched import.

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