The default branch is now 'main'

The default branch is now 'main'

Updated the HEAD symbolic reference.
Updated gitsigure checkref.
Updated hooks/post-receive irkerhook
Updated buildbot elfutils_[snapshots_]scheduler and elfutils_gitpoller

If you still have a checkout that refers to master and you do a git
pull you might get:

  Your configuration specifies to merge with the ref
  'refs/heads/master' from the remote, but no such ref was
  fetched.

Make sure to rename your branch to main and set the new upstream:

$ git branch -m master main
$ git branch --unset-upstream
$ git branch -u origin/main

Signed-off-by: Mark Wielaard <mark@klomp.org>