diff --git a/commit.c b/commit.c
index 55b1c8d2f8..28bb5ce029 100644
--- a/commit.c
+++ b/commit.c
@@ -1988,9 +1988,14 @@ void commit_stack_init(struct commit_stack *stack)
 	stack->nr = stack->alloc = 0;
 }
 
+void commit_stack_grow(struct commit_stack *stack, size_t extra)
+{
+	ALLOC_GROW(stack->items, st_add(stack->nr, extra), stack->alloc);
+}
+
 void commit_stack_push(struct commit_stack *stack, struct commit *commit)
 {
-	ALLOC_GROW(stack->items, stack->nr + 1, stack->alloc);
+	commit_stack_grow(stack, 1);
 	stack->items[stack->nr++] = commit;
 }
 
diff --git a/commit.h b/commit.h
index 7c01a76425..79a761c37d 100644
--- a/commit.h
+++ b/commit.h
@@ -388,6 +388,7 @@ struct commit_stack {
 #define COMMIT_STACK_INIT { 0 }
 
 void commit_stack_init(struct commit_stack *);
+void commit_stack_grow(struct commit_stack *, size_t);
 void commit_stack_push(struct commit_stack *, struct commit *);
 struct commit *commit_stack_pop(struct commit_stack *);
 void commit_stack_clear(struct commit_stack *);