git-mergetool to be used for rebases as well?

An internal user report running on origin/next:

    $ git pull
        From .
         * branch                ... -> FETCH_HEAD
        First, rewinding head to replay your work on top of it...
        Applying: ...
        Applying: ...
        Using index info to reconstruct a base tree...
        CONFLICT (content): ....
        error: Failed to merge in the changes.
        Patch failed at 0002...
        The copy of the patch that failed is found in: .git/rebase-apply/patch

        When you have resolved this problem, run "git rebase --continue".
        If you prefer to skip this patch, run "git rebase --skip" instead.
        To check out the original branch and stop rebasing, run "git rebase --abort".
    $ git status
        rebase in progress; onto ...
        You are currently rebasing branch '...' on '...'.

        Changes to be committed:
                modified:   ...

        Unmerged paths:
                both modified:   ...

    $ git mergetool
        No files need merging
    $ git diff <file name>
        diff --cc <file name>
        index ...
        --- a/file
        +++ b/file
        @@@ ...
          content
        ++<<<<<<< HEAD
         +  content
        ++=======
        +   content
        ++>>>>>>> other commit
        content


The mergetool used to work apparently, but stopped for rebases.
I noticed in neither t7610-mergetool.sh nor any rebase test the combination of
rebase and mergetool is tested.

Stefan