git prune-branches

@kevgo please review

+ Changed a couple git helpers to take in parameters instead of operating on the current branch
+ Set the default branch on the remote repo to main (needed as the git prune-branches feature specs were deleting the master branch (since its not main) and you cannot delete the HEAD branch on the remote repo.
I think we should distinguish several cases here:

* __empty feature branches: __fresh cut branches with no commits at all (point to the same commit as main). I think we shouldn't delete those branches of other users, since they most likely have just created them.

* __stale feature branches: __branches that have been merged into main a while ago, and are behind main now. We can prune them. Ideally only if they are older than a few days. 

* __outdated feature branches: __branches with their own commits, that aren't up to date with main, but less than 3 weeks old. We shouldn't prune them.

* __abandoned feature branches: __branches with their own commits that are older than 3 weeks. We should offer to prune them.

What do you think about this? The "3 weeks" should probably be configurable.