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).
We should not "slow down" the combo box.