bug#41764: `make authenticate` fails to find the keyring branch

[Leo Famulari]

I just tried pushing for the first time since installing the new
pre-push hook that runs `make authenticate`.

This failed with the following error:

Git error: cannot locate remote-tracking branch 'keyring'

However, `git branch --all` includes "remotes/origin/keyring".

After I did `git checkout origin/keyring`, it worked.

Let's update the manual section Commit Access with the recommended way
to make this branch accessible to `make authenticate`. Maybe it should
even do it automatically?