Add the "cmrel update-release-branch" command

[maelvls]

This PR creates a new command cmrel update-release-branch, and the goal is to automate this part of our release-process.

The cmrel update-release-branch --branch release-1.4 does the following:

• if the branch release-1.4 does not exist, create it,
• if the branch release-1.4 already exists, merge --ff-only master and push it.

Note that cmrel update-release-branch currently does not know about the "code freeze": cmrel update-release-branch must not be run during a code freeze, since merged PRs myst be cherry picked during that period.

The part of release process in question:

3. Create or update the release branch:

   • (initial alpha only) Create the release branch:

     # Must be run from the cert-manager repo folder.
     git fetch --all
     git checkout -b release-1.0 origin/master

   • (subsequent alpha, beta and final releases only); You need to
     update the release branch with the latest commits from the master
     branch, as follows:

     # Must be run from the cert-manager repo folder.
     git fetch --all
     git branch --force release-1.0 origin/release-1.0
     git checkout release-1.0
     git merge --ff-only origin/master

     Patch releases do not require this git merge --ff-only step, since
     the merge is done by opening a PR using the /cherry-pick release-1.0 command.

4. Push the new or updated release branch:

   1. Check that the origin remote is correct. To do that, run the following
      command and make sure it returns
      the upstream https://github.com/jetstack/cert-manager.git:

      # Must be run from the cert-manager repo folder.
      git remote -v | grep origin

   2. Push the release branch:

      # Must be run from the cert-manager repo folder.
      git push --set-upstream origin release-1.0

      (initial alpha only): git push will only work if you have the
      write or admin GitHub permission on the cert-manager repo to create
      or push to the branch, see prerequisites.

[jetstack-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: maelvls

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [maelvls]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[wallrj]

This PR title is wrong I think.

[maelvls]

Oops, I think I mixed two of my PRs. Thanks for spotting that!

[jetstack-bot]

@maelvls: PR needs rebase.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.