`git svn branch` does not respect authors file

I have a git repository linked to a svn repository with git-svn,
including a branch path configuration and an authorsfile for svn
username -> email mapping.


When running `git svn branch new_branch`, git-svn:
* Creates a svn commit creating a new svn branch
* Creates a local git commit linked to this svn commit
The svn commit is correctly generated, but the corresponding git
commit has a bad author: the author is just the svn username.


Re-fetching the svn repository then properly applies the authors file,
but ideally it would do it directly when creating the branch.