commit-graph: don't peek into `struct lock_file`

Similar to the previous commit, avoid peeking into the `struct
lock_file`. Use the lock file API instead.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -1694,8 +1694,8 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 	} else {
 		hold_lock_file_for_update_mode(&lk, ctx->graph_name,
 					       LOCK_DIE_ON_ERROR, 0444);
-		fd = lk.tempfile->fd;
-		f = hashfd(lk.tempfile->fd, lk.tempfile->filename.buf);
+		fd = get_lock_file_fd(&lk);
+		f = hashfd(fd, get_lock_file_path(&lk));
 	}
 
 	chunks[0].id = GRAPH_CHUNKID_OIDFANOUT;
@@ -1833,7 +1833,7 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 		result = rename(ctx->graph_name, final_graph_name);
 
 		for (i = 0; i < ctx->num_commit_graphs_after; i++)
-			fprintf(lk.tempfile->fp, "%s\n", ctx->commit_graph_hash_after[i]);
+			fprintf(get_lock_file_fp(&lk), "%s\n", ctx->commit_graph_hash_after[i]);
 
 		if (result) {
 			error(_("failed to rename temporary commit-graph file"));