[git-debrebase upstream-combine . PIECE[ PIECE...]: new upstream]
[git-debrebase anchor: new upstream NEW-UPSTREAM-VERSION, merge]
[git-debrebase: new upstream NEW-UPSTREAM-VERSION, changelog]
+ [git-debrebase: export and commit patches]
[git-debrebase convert-from-gbp: drop patches]
[git-debrebase anchor: declare upstream]