Add support for scrubbing obsolete pull request branches, and make pull-requests-prep call it.
Add support for scrubbing obsolete pull request branches, and make
pull-requests-prep call it.

MaximilianMeister/crowbar,rsalevsky/crowbar,SUSE-Cloud/crowbar,tserong/crowbar,nkrinner/crowbar,aplanas/crowbar,toabctl/crowbar,rsalevsky/crowbar,crowbar/crowbar,rhafer/crowbar,dirkmueller/crowbar,rhafer/crowbar,rhafer/crowbar,mmnelemane/crowbar,SUSE-Cloud/crowbar,dirkmueller/crowbar,aspiers/crowbar,vwallfahrer/crowbar,toabctl/crowbar,aspiers/crowbar,mmnelemane/crowbar,tserong/crowbar,aplanas/crowbar,MaximilianMeister/crowbar,vwallfahrer/crowbar,crowbar/crowbar,nkrinner/crowbar