rev-tree now handles commit problems better

This fixes possible crashes in case of broken commit tree, and makes
rev-tree die in case it cannot parse a given commit.

rev-tree.c

@@ -51,11 +51,11 @@ void process_commit(unsigned char *sha1)
 	struct commit_list *parents;
 	struct commit *obj = lookup_commit(sha1);
 
-	if (obj->object.parsed)
+	if (obj && obj->object.parsed)
 		return;
+	if (!obj || parse_commit(obj))
+		die("unable to parse commit (%s)", sha1_to_hex(sha1));
 
-	parse_commit(obj);
-	
 	parents = obj->parents;
 	while (parents) {
 		process_commit(parents->item->object.sha1);