$ git checkout master
$ git checkout -b start-anew
$ git reset --soft 2.12_002
$ git commit -m "Rewind to tag 2.12_002"
# hack, hack, hack
$ git add ...
$ git commit -m "Slight touchups to what was 2.12_002"
$ git checkout master
$ git merge start-anew
$ git tag "2.12_005"