cmd/update*: more fixes to allow migration from master to main.

[MikeMcQuaid]

We're not migrating Homebrew/brew to main yet but this will allow us to ensure we have a brew update ready in future when we're ready to do this.

Tested by adjusting default branch on MikeMcQuaid/brew to main.

Again @homebrew/maintainers would really appreciate a very thorough review here.

May also take an approach like in Homebrew/actions#702 in Homebrew/brew to be extra conservative here for a wee while.

[Bo98]

Seems all fine

[Bo98]

May also take an approach like in Homebrew/actions#702 in Homebrew/brew to be extra conservative here for a wee while

Something we've seen many times before is there's external CI providers that do "frozen" base images. This is especially common on those that need to use older macOS images. So there are users that will still be today doing brew update from e.g. 3.0.x to 4.5.6.

Ideally we'd want to keep that update path working. Syncing would probably work best but if we're against that (or have a desire to drop syncing at some point) then a locked branch allowing a two-stage update is a possible alternative.

brew update-test should cover most cases I think though - assuming it has sufficient coverage of updating master/main rather than just tags.

[MikeMcQuaid]

Ideally we'd want to keep that update path working.

We should keep it working for a while (e.g. 1 year max). We should not aim to keep it working in perpetuity.

[Rylan12]

This will fetch PR branch refs too, right? Is that something we want to avoid?

[MikeMcQuaid]

It will but doesn't seem like a big problem either way. I can't seem to find a way to fetch only main and/or master, no other refs and not error if one is missing.