message: if heas is invalid show abort message
diff: @@ -1412,7 +1412,10 @@ void RepoView::continueRebase() {
  void RepoView::rebase(const git::AnnotatedCommit &upstream, LogEntry *parent) {
  git::Branch head = mRepo.head();
  - Q_ASSERT(head.isValid());
  + if (!head.isValid()) {
  + addLogEntry(tr("Invalid head."), tr("Abort"), parent);
  + return;
  + }
  mRebase = parent;
  