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/libostree/ostree-repo.h

@@ -37,6 +37,8 @@ GType ostree_repo_get_type (void);
 
 OstreeRepo* ostree_repo_new (GFile *path);
 
+OstreeRepo* ostree_repo_new_default (void);
+
 gboolean      ostree_repo_check (OstreeRepo  *self, GError **error);
 
 GFile *       ostree_repo_get_path (OstreeRepo  *self);