Show progress for Filtering Content during clone [SG-15033]

When cloning a repository, SmartGit shows a nice progress-bar, except for the step "Filtering content", which is usually quite fast, but in my example, it took several minutes. Not knowing, whether SmartGit hung up or not, I force-closed the application and cloned the repo from the command line.

It would be great to show the progress and that something is still going on while the clone operation is filtering content.

instead, the UI gets stuck here for a very long time: