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

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[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.

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