Check if branch already exists

[dragonraid]

If you rerun the job, before deleting branch created by previous run, it fails on:

hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

    at GitExecutorChain.onFatalException (/app/node_modules/simple-git/src/lib/runners/git-executor-chain.js:60:87)
    at GitExecutorChain.<anonymous> (/app/node_modules/simple-git/src/lib/runners/git-executor-chain.js:51:28)
    at Generator.throw (<anonymous>)
    at rejected (/app/node_modules/simple-git/src/lib/runners/git-executor-chain.js:6:65)
    at processTicksAndRejections (internal/process/task_queues.js:93:5) {
  task: {
    commands: [ 'push', 'origin', 'update/helmfile', '--verbose', '--porcelain' ],
    format: 'utf-8',
    parser: [Function (anonymous)]
  }
}