🟢 ☂️ rename "master" branch to "main"

[pq]

Following Dart, Flutter, Chrome et al., and inspired by conversations such as this one, I'd like to migrate the linter to "main". Though we don't use gclient for development, the SDK (which has the linter in DEPS does) and I understand from @athomas that may complicate things and we should wait until the SDK is fully moved over. Holding here until we get the green light.

All: please feel free to chime in on this tracking issue if you have thoughts or concerns.

(For reference, here's the Changing the default branch doc and a GitHub renaming guide.)

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Punch list of things to update when we go ahead:

• CONTRIBUTING (Checking Dartium setting "enable debugging" under "Manage launches" display blank page #3518)
• .github/workflows (Checking Dartium setting "enable debugging" under "Manage launches" display blank page #3518)
• README and scoring scripts (for SDK and Flutter links) (Checking Dartium setting "enable debugging" under "Manage launches" display blank page #3518)
• doc/** (Checking Dartium setting "enable debugging" under "Manage launches" display blank page #3518)
• (internal) update copybara scripts (cl/460768093)

[athomas]

@pq I think we should try to do this. I can't see any reason why it won't work:

• DEPS doesn't contain information about the branch the hash is on.
• We've previously seen that gclient sync was able to pull changes even from branches used just for PRs (not that that's a good thing, but it's further evidence that the branch name doesn't matter).
• We already pull in pkg:lints into the SDK which uses main as a default branch.

When you do decide to move forward, please keep an eye on the console for a few hours and be prepared to re-create a master branch if purpleness starts to show up (it should be sufficient to create another branch called master that has all the commits from main rather than renaming main to master). There's a risk some caches on the bots might be affected by this change (though I think that risk is very low).

[athomas]

@pq See also email thread about potential internal infra impact.

[pq]

@iinozemtsev : do you know if any of your scripts (e.g., find_violations?) will need updating?

[iinozemtsev]

I think we'll only need to update copybara config once it happens, I can do this

[pq]

Super. Thanks @iinozemtsev!

In that case, I don't see anything significant holding this back and may look into doing the change next week.