repo: Make read_commit spit out a resolved commit ref as well

read_commit resolves the ref to a commit, and a lot of consumers want
the resolved commit for their own purposes; this prevents them from
calling resolve_rev themselves.

https://bugzilla.gnome.org/show_bug.cgi?id=707727

src/ostree/ot-builtin-cat.c

@@ -78,7 +78,7 @@ ostree_builtin_cat (int argc, char **argv, OstreeRepo *repo, GCancellable *cance
     }
   rev = argv[1];
 
-  if (!ostree_repo_read_commit (repo, rev, &root, NULL, error))
+  if (!ostree_repo_read_commit (repo, rev, &root, NULL, NULL, error))
     goto out;
 
   stdout_stream = g_unix_output_stream_new (1, FALSE);