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-cmdprivate.c

@@ -47,6 +47,7 @@ ostree_cmd__private__ (void)
   static OstreeCmdPrivateVTable table = {
     impl_ostree_generate_grub2_config,
     _ostree_repo_static_delta_dump,
+    _ostree_repo_static_delta_query_exists,
     _ostree_repo_static_delta_delete
   };