checkout: use target path; don't assume workdir

We're not necessarily checking out into the working directory.  We could
be checking out into an arbitrary location.  Ensure that when we are
writing conflict data that we do it in the checkout target.

src/checkout.c

@@ -2032,7 +2032,7 @@ static int checkout_merge_path(
 	const char *our_label_raw, *their_label_raw, *suffix;
 	int error = 0;
 
-	if ((error = git_buf_joinpath(out, git_repository_workdir(data->repo), result->path)) < 0)
+	if ((error = git_buf_joinpath(out, data->opts.target_directory, result->path)) < 0)
 		return error;
 
 	/* Most conflicts simply use the filename in the index */