If a runner is re-used, the presence of `TMP_DIR` will cause the script
to fail. To be safe, we'll also remove `SVN_DIR` to prevent conflicts,
even though the directories are timestamped.

Fixes #6

svn lacks a rename/replace function, requiring first that an existing
tag be deleted before an update is copied into place. svn is, fortunately,
generally smart enough to sort out the history and show only the updated
files in the resulting commit.