With git, how do I save a topic branch after the parent remote branch was rebased?

Question

I had a local topic branch based off a remote branch on the git repo:

1 - 2 - 4 - 5 - 9 - 11  master
      \
        3 - 6 - 8 - 12  remote branch
              \
                7 - 10  my topic

The remote branch was recently rebased off master:

1 - 2 - 4 - 5 - 9 - 11  master
                       \
            ?            3' - 6' - 8' - 12'  remote branch
              \
                7 - 10  my topic

I would like to rebase my topic branch off the head of the remote branch so that I can eventually merge my changes back into the remote. However, because the hash of the parent commit has changed, trying a simple

$ git rebase origin/remote_branch

results in a number of conflicts in files I've never touched.

Is there anything I can do to clean up these branches?

Answer 1

Simply rebasing on origin/remote_branch is confusing for git because it tries to rebase outdated commits on the tip of remote_branch, which leads to duplicate commits and conflicts.

Instead you must plug off and on, to the new point commit related to the sixth one, using the --onto option

git rebase --onto <6'-SHA> <6-SHA> my-topic

<6'-SHA> and <6-SHA> are respectively the new and old ids of the commit where my-topic was created.