object: drop parsed_object_pool->commit_count

14ba97f8 (alloc: allow arbitrary repositories for alloc functions,
2018-05-15) introduced parsed_object_pool->commit_count to keep count of
commits per repository and was used to assign commit->index.

However, commit-slab code requires commit->index values to be unique
and a global count would be correct, rather than a per-repo count.

Let's introduce a static counter variable, `parsed_commits_count` to
keep track of parsed commits so far.

As commit_count has no use anymore, let's also drop it from the struct.

Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

blob.c

@@ -10,7 +10,7 @@ struct blob *lookup_blob(struct repository *r, const struct object_id *oid)
 	struct object *obj = lookup_object(r, oid);
 	if (!obj)
 		return create_object(r, oid, alloc_blob_node(r));
-	return object_as_type(r, obj, OBJ_BLOB, 0);
+	return object_as_type(obj, OBJ_BLOB, 0);
 }
 
 int parse_blob_buffer(struct blob *item, void *buffer, unsigned long size)