[Snowball-discuss] git "master" branches renamed to "main"

[Olly Betts]

I have renamed the "master" branches of all the snowball git repos to
"main" (which will be the default branch name in git 3.0 and already is
the default in some of the hosted git forges).

This is a change that's useful to make at some point so that our default
branch name matches typical modern git usage.  Also the snowball-website
repo was already using "main", so this resolves an inconsistency between
our own repos.

New clones of any of the repos from github should use branch "main"
automatically.

If you have an existing local clone of any of the repos you can update
it for this change like so (assuming your remote is named "origin",
which it will be by default):

    git branch -m master main
    git fetch origin
    git branch -u origin/main main
    git remote set-head origin -a

(If you visit the repo on the web at github it shows a pop up with
those commands, though I don't know how long it does that for.)

I've updated CI, and open PRs should have been automatically retargetted
to main.

Please report if you find anything that's not working due to this change.

Cheers,
    Olly