Make it possible to abort loading branches/tags thread

Try to improve [issue #2187](https://code.google.com/p/tortoisegit/issues/detail?id=2187) after saying something in [this comment](https://code.google.com/p/tortoisegit/issues/detail?id=2231#c3) of [issue #2231](https://code.google.com/p/tortoisegit/issues/detail?id=2231).
Yap. It is just for testing. Since the issue 1951 was fixed, loading refs is fast. 
So, I do it on purpose. :)