Works for me:

reedy@ubuntu64-web-esxi:/var/www/wiki/mediawiki/extensions$ git pull
Already up-to-date.
reedy@ubuntu64-web-esxi:/var/www/wiki/mediawiki/extensions$ git submodule update Persona
Submodule path 'Persona': checked out '655d019b84689e37b3fb1018600f629c657fb575'

Delete the extension folder and then re-pull/submodule update

Also, git head is now mediawiki 1.21

Thanks for checking, error thus seems to be on our side (1.21: setting git checkout REL1_20, that is not a problem). I rechecked and the error still occurs.

If you know: Does a less drastic option than deleting all extensions exist? (Deleting all is a hassle, because a number of extensions not yet available in git as well as some local ones are interspersed with the git ones.) I had already tried to delete the Persona folder, of course, but that does not help.