delta: Add --if-not-exists option

I often want to have "idempotent" systems that iterate to a known
state.  If after generating a commit, the system is interrupted, I'd
like the next run to still generate a delta.  But we don't want to
regenerate if one exists, hence this option.

Closes: #375
Approved by: jlebon

src/libostree/ostree-repo-static-delta-private.h

@@ -190,6 +190,13 @@ _ostree_delta_compute_similar_objects (OstreeRepo                 *repo,
                                        GCancellable               *cancellable,
                                        GError                    **error);
 
+gboolean
+_ostree_repo_static_delta_query_exists (OstreeRepo                 *repo,
+                                        const char                 *delta_id,
+                                        gboolean                   *out_exists,
+                                        GCancellable               *cancellable,
+                                        GError                    **error);
+
 gboolean
 _ostree_repo_static_delta_dump (OstreeRepo                 *repo,
                                 const char                 *delta_id,