Prevent pull-to-update-repo queueing
When you pull down to update repo once, the process starts as expected. But if you do it again, accidentally or otherwise, the repo update starts again immediately after the previous one ends. This way, depending on how long the repo update takes, you can queue several updates, which is pointless.
F-Droid should ignore the pull down gesture if a repo update is already taking place. A new update process should start only if the pull down gesture is made when there is no previous update taking place.