git-svn is a git command that allows using git to interact with Subversion repositories.git-svn is part of git, meaning that is NOT a plugin but actually bundled with your git installation. SourceTree also happens to support this command so you can use it with your usual workflow.
Reference: http://git-scm.com/book/en/v1/Git-and-Other-Systems-Git-and-Subversion
You need to create a new local copy of the repository with the command