How to convert SVN tags to git tags?

After converting from SVN to git using git svn init + git svn fetch, my SVN tags appear as remote branches in git. I find many suggestions to convert these to git tags by calling git tag -a <tag-name> <branch-name> but this gives something like this

               C4
               |
               V
    T -> C3 -> C2
               |
               V
               C1

that is, I still have a branch with only 1 commit C3 and I have a tag T pointing to that commit. Also, T will have a current timestamp. I'd prefer to get

               C4
               |
               V
          T -> C2
               |
               V
               C1

where T takes the commit message and date from C3. How can I achieve that?

Steve