main: Code cleanup by passing OstreeRepo * directly to builtins

It turns out every builtin (with one special exception) that takes a
repo argument did the same thing; let's just centralize it.  The
special exception was "ostree init --repo=foo" where foo is expected
to *not* actually be a repo.  In that case, simply skip the
ostree_repo_check() invocation.

https://bugzilla.gnome.org/show_bug.cgi?id=706762

src/ostree/ot-builtin-cat.c

@@ -55,13 +55,12 @@ cat_one_file (GFile         *f,
 }
 
 gboolean
-ostree_builtin_cat (int argc, char **argv, GFile *repo_path, GCancellable *cancellable, GError **error)
+ostree_builtin_cat (int argc, char **argv, OstreeRepo *repo, GCancellable *cancellable, GError **error)
 {
   GOptionContext *context;
   gboolean ret = FALSE;
   int i;
   const char *rev;
-  gs_unref_object OstreeRepo *repo = NULL;
   gs_unref_object GOutputStream *stdout_stream = NULL;
   gs_unref_object GFile *root = NULL;
   gs_unref_object GFile *f = NULL;
@@ -72,10 +71,6 @@ ostree_builtin_cat (int argc, char **argv, GFile *repo_path, GCancellable *cance
   if (!g_option_context_parse (context, &argc, &argv, error))
     goto out;
 
-  repo = ostree_repo_new (repo_path);
-  if (!ostree_repo_check (repo, error))
-    goto out;
-
   if (argc <= 2)
     {
       ot_util_usage_error (context, "An COMMIT and at least one PATH argument are required", error);