Summary:
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.

Fixes T13293.

Test Plan:
`arc land` without errors (or manually creating a `trunk` branch) from a
checkout made with Git 2.18.0 (verified this manually on a non-`-T`
checkout as well).

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T13293

Differential Revision: https://secure.phabricator.com/D19681