Migrate ALL providers to expecting main branch, not just GitHub, and announce the change

The work done in https://github.com/jupyterhub/binderhub/pull/1172 means that binderhub now looks for the main branch if a ref isn’t provided, rather than master. This is probably a change we should roll out across all providers but I, at least, am unfamiliar with where these other communities are in this process and this change will be breaking previously established behaviour, possibly disrupting a lot of users. Therefore, I think we should have some form of “announcement” (blog post, twitter thread…) ready to go when we do make this change, just so users aren’t blind-sided and they can find information on how to fix their links.

Note: This change does not affect gists over which the user has no control of the ref naming.

TODOs

• Draft a blog post/twitter thread to announce the change
• Change the behaviour of the placeholder variable added in PR #1172

placeholder = "main";
if (provider === "gist") {
    placeholder = "master";
}

[betatim]

Take a look at #843 which was closed by #895. Maybe we can hoist this out of the Git repo provider and use it for GitHub and GitLab as well?

[minrk]

Yes, this is related to #895 which also uses ls-remote to resolve refs. The addition of --symref HEAD is what lets us find the branch name associated with HEAD.

But this is even easier than I thought! HEAD is already a valid ref, so it’s working already and we should just make our default ref HEAD instead of a branch name, and our existing APIs already work:

https://mybinder.org/v2/gh/binderhub-ci-repos/requirements/HEAD

This is strictly better than resolving the default branch name and using that in the URL, because it allows creating a stable link that will work across default branch changes without changes to the link.

Need to test with all providers, but I think it’ll work.