Renaming HEAD git branches

[Guillem Jover <guillem@debian.org>]

Hi!

I'll start renaming the HEAD git branches to main for all dpkg.org git
repos. The dpkg one I'll defer until I've released 1.20.8 and opened
HEAD for 1.21.x.

The rationale is that, while I'm far removed from a direct environment
where the current name could be interpreted as offensive or triggering,
language defines our thought and society. It's an easy gesture to do
when this can affect other people. The provenance of the current name
is not great either. And the new name is shorter, more descriptive,
various hosting sites are already defaulting to it, and newer git
releases will default to it soonish.

To update your work directories you can do something like the
following once the main branch appears:

  ,---
  git fetch origin
  git remote set-head origin main
  git remote prune origin
  git branch -u origin/main
  git branch -m main
  git remote -v
  git branch -a
  `---

Thanks,
Guillem