commit-graph: load a root tree from specific graph

When lazy-loading a tree for a commit, it will be important to select
the tree from a specific struct commit_graph. Create a new method that
specifies the commit-graph file and use that in
get_commit_tree_in_graph().

Signed-off-by: Derrick Stolee <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: derrickstolee

commit-graph.c

@@ -362,14 +362,20 @@ static struct tree *load_tree_for_commit(struct commit_graph *g, struct commit *
 	return c->maybe_tree;
 }
 
-struct tree *get_commit_tree_in_graph(const struct commit *c)
+static struct tree *get_commit_tree_in_graph_one(struct commit_graph *g,
+						 const struct commit *c)
 {
 	if (c->maybe_tree)
 		return c->maybe_tree;
 	if (c->graph_pos == COMMIT_NOT_FROM_GRAPH)
-		BUG("get_commit_tree_in_graph called from non-commit-graph commit");
+		BUG("get_commit_tree_in_graph_one called from non-commit-graph commit");
+
+	return load_tree_for_commit(g, (struct commit *)c);
+}
 
-	return load_tree_for_commit(commit_graph, (struct commit *)c);
+struct tree *get_commit_tree_in_graph(const struct commit *c)
+{
+	return get_commit_tree_in_graph_one(commit_graph, c);
 }
 
 static void write_graph_chunk_fanout(struct hashfile *f,