svn: Honor --prefix option in init without --stdlayout

Most users who type

  git svn init file:///tmp/repo --prefix=my-svn/

would expect the root of the svn repository to be tracked by
refs/remotes/my-svn/git-svn.

Acked-by: Eric Wong <normalperson@yhbt.net>

git-svn.perl

@@ -3317,7 +3317,8 @@ sub _new {
 		$repo_id = $Git::SVN::default_repo_id;
 	}
 	unless (defined $ref_id && length $ref_id) {
-		$_[2] = $ref_id = $Git::SVN::default_ref_id;
+		$_prefix = '' unless defined($_prefix);
+		$_[2] = $ref_id = $_prefix . $Git::SVN::default_ref_id;
 	}
 	$_[1] = $repo_id;
 	my $dir = "$ENV{GIT_DIR}/svn/$ref_id";