Perform auto-deploy picking for Omnibus

2ed9b439 - Alias AutoDeploy::Version#stable_branch to auto_deploy_branch

219b83fa - Reduce duplication in auto_deploy:pick task

We're going to be adding Omnibus as a third project for auto-deploy picking, and would have this task especially repetitive, so we're refactoring it out into a method.

c9a748c8 - Perform auto-deploy picking for Omnibus

05ca5c96 - Use cherry_pick_result helper for release:merge

Closes https://gitlab.com/gitlab-org/release/framework/issues/379