This might not be the same cause or solution as [[git_test_receive_wrapper_fails]] so filing
it separately.

Anon git push is broken again

    remote: error: cannot lock ref 'HEAD': Unable to create '/home/b-ikiwiki/source.git/./HEAD.lock': Permission denied
    To git://git.ikiwiki.info/
     ! [remote rejected]     master -> master (failed to update ref)

*—[[Jon]], 2020-10-06*

> It does not seem related to the other problem. I don't understand how it
> could have broken in a new way since I fixed it before. git has not been
> upgraded in the meantime. This is affecting more than one site, and the
> permissions do not seem obviously broken.
> 
> Since this is now seeming so fragile -- after working for about a decade,
> it's broken twice in a matter of months -- I'm questioning whether it's
> worth trying to keep the feature working. --[[Joey]]