+ # $new_upstream is either the single upstream commit, or the
+ # combined commit we just made. Either way it will be the
+ # "upstream" parent of the breakwater special merge.
+
+ read_tree_subdir 'debian', "$old_bw:debian";
+
+ # index now contains the breakwater merge contents
+ $new_bw = make_commit [ $old_bw, $new_upstream ],
+ [ "Update to upstream $new_upstream_version",
+ "[git-debrebase breakwater: new upstream $new_upstream_version, merge]",
+ ];
+
+ # Now we have to add a changelog stanza so the Debian version
+ # is right.
+ die if unlink "debian";
+ die $! unless $!==ENOENT or $!==ENOTEMPTY;
+ unlink "debian/changelog" or $!==ENOENT or die $!;
+ mkdir "debian" or die $!;
+ open CN, ">", "debian/changelog" or die $!;
+ my $oldclog = git_cat_file ":debian/changelog";
+ $oldclog =~ m/^($package_re) \(\S+\) / or
+ fail "cannot parse old changelog to get package name";
+ my $p = $1;
+ print CN <<END, $oldclog or die $!;
+$p ($new_version) UNRELEASED; urgency=medium
+
+ * Update to new upstream version $new_upstream_version.
+
+ --
+
+END
+ close CN or die $!;
+ runcmd @git, qw(update-index --add --replace), 'debian/changelog';
+
+ # Now we have the final new breakwater branch in the index
+ $new_bw = make_commit [ $new_bw ],
+ [ "Update changelog for new upstream $new_upstream_version",
+ "[git-debrebase: new upstream $new_upstream_version, changelog]",
+ ];
+ };
+
+ # we have constructed the new breakwater. we now need to commit to
+ # the laundering output, because git-rebase can't easily be made
+ # to make a replay list which is based on some other branch
+
+ update_head_postlaunder $old_head, $old_laundered_tip,
+ 'launder for new upstream';
+
+ my @cmd = (@git, qw(rebase --onto), $new_bw, $old_bw, @ARGV);
+ runcmd @cmd;
+ # now it's for the user to sort out
+}
+
+sub cmd_gbp2debrebase () {
+ badusage "needs 1 optional argument, the upstream" unless @ARGV<=1;
+ my ($upstream_spec) = @ARGV;
+ $upstream_spec //= 'refs/heads/upstream';
+ my $upstream = git_rev_parse $upstream_spec;
+ my $old_head = get_head();
+
+ my $upsdiff = get_differs $upstream, $old_head;
+ if ($upsdiff & D_UPS) {
+ runcmd @git, qw(--no-pager diff),
+ $upstream, $old_head,
+ qw( -- :!/debian :/);
+ fail "upstream ($upstream_spec) and HEAD are not identical in upstream files";
+ }