Deal with the remaining branches' Git history and interdependencies

After the first test run of history rewriting: survey the branches that are left, identify how well their history needs to be kept, if at all, rebase each remaining topic branch on top of the current main branch it is based upon; if needed, preserve more history relationships between branches that what rebasing above alone would do.

To work on this without waiting for #8617 (closed) and for the first trial rewrite, one can list the branches that have not been merged into devel this way:

git checkout devel && git branch -r --no-merged \
  | perl -n -E 'if (m,^\s+origin/,) { s,^(\s+origin/),, ; print}'

Blueprint: https://tails.boum.org/blueprint/rewrite_Git_history/

Parent Task: #6277 (closed)

Related issues

• Blocked by #8619 (closed)
• Blocks #8629 (closed)

Original created by @intrigeri on 8618 (Redmine)