GPU memory leak during large source control operations

We've seen some reports of GPU memory leaks during source control operations caused by the animation of the throbber widget, potentially leading to a crash if GPU memory runs out. The crash has been seen in Git and SVN, and can be avoided by removing FScopedSourceControlProgress::Tick. This will prevent Slate from being ticked during the operation, but will prevent the throbber widget from animating.

1. Connect to source control via Git or SVN
2. Copy a large number of assets to trigger a long source control operation
3. Watch GPU memory increase during the operation