Add "stamp" action for nightly integration branch following

Historically we've used a nightly-master branch in some projects that a bot fast-forwards to master each night at a given time.

We also had the same thing for next, and the latter has become the stage action's tag_stage operation. It produces:

• refs/stage/master/nightly/latest
• refs/stage/master/nightly/CCYY/MM/DD

based on the value of refs/stage/master/head.

We need an equivalent of this for following an integration branch without considering the stage. I propose a "stamp" operation that can be scheduled each night. It would produce:

• refs/stamp/master/nightly/latest
• refs/stamp/master/nightly/CCYY/MM/DD

based on the value of refs/heads/master.