The current code assumes git-svn is always working from a remote called
`trunk`, but if the repository is initialized without the `-T` option it
will instead be called `git-svn`, and if `--prefix` is used (which is
set by default to `origin/` in Git 2+) the remote name will have the
specified prefix as well.
Instead, look at the `fetch` target refspec set in the git-svn config.