Pull / push - use fetched remote branch info unless explicitly requested

In 1.3 the push/pull behaviour from the sidebar was sped up by not checking the remote if there were tracked branches already.

Using the push/pull toolbar buttons though still waits for the remote refresh.

While there are cases when the remote branch information is out of date, in practice it's quite rare. It would be enough to simply grab the remote branches from the locally fetched list and provide a fetch button on the pull/push dialog that users could click if they needed to.