core: Allow resolving e.g. "origin/master" as a rev

Also:

* Disallow .. in revs to avoid looking outside where we should be
* Allow resolving empty revs

src/libostree/ostree-repo.h

@@ -86,6 +86,7 @@ gboolean      ostree_repo_store_object_trusted (OstreeRepo   *self,
 
 gboolean      ostree_repo_resolve_rev (OstreeRepo  *self,
                                        const char  *rev,
+                                       gboolean     allow_noent,
                                        char       **out_resolved,
                                        GError     **error);