renaming the default branch to 'main'

[Gaetan Bisson]

[2020-06-15 13:01:04 -0700] Dirk Hohndel via subsurface:
> I strongly believe that words matter. And that sometimes it's the small things where we can show respect.

Shall we rename divemaster too? As an explicit GUI label it gets much
more user exposure than the default branch name. I would suggest dive
leader but it is probably just as offensive due to its resemblance to
nazi titles.

Cheers.

-- 
Gaetan