Add tags to commits pushed to epoch branches

[clopez]

This epoch/ branches are used to trigger the CI runs. However, identifying which commit of that branch triggered the run on a given day of the past is not trivial. There are several variables at play, like the timezone of the bot running the action which complicates this.

For example on #26097 I was unable to identify the commit that triggered the taskcluster run on epochs/daily for october 12.

So I think it is a good idea that we start tagging the commits pushed to this branches.

The format for the tags proposed here is the name of the branch with the date (year-month-day_hour). Example:

epochs/daily/2020-10-14_22H
epochs/six_hourly/2020-10-14_22H
epochs/three_hourly/2020-10-14_22H
epochs/twelve_hourly/2020-10-14_22H
epochs/weekly/2020-10-14_22H

This will be helpful to exactly identify which commits where pushed to the epoch/ branches on a given date.

[clopez]

BTW, I'm really only interested in tagging the daily and weekly ones.
If you think tagging the hourly ones is too much noise (too much tags) then I can modify this to only tag the weekly and dailies.

[clopez]

BTW, the failure on the CI seems totally unrelated to this commit

[gsnedders]

Personally, I'm in favour of this, but should probably check with others? (ping @jgraham // @stephenmcgruer)

[stephenmcgruer]

Thanks Carlos, I think this is an improvement too! We have also felt the pain of finding which commits were epoch pushes, even on the three_hourly and six_hourly branches. We tag nearly ever commit today with the merge_pr_xyz setup (https://github.com/web-platform-tests/wpt/tags), so this will be no spammier than that...

[clopez]

Thanks!