On login, look up username by OpenID identifier.
On login, look up username by OpenID identifier.

On login, first try to look up the username by OpenID identifier, using
get_user from branch tmp-non-ff-rewrite. Only create a (new) username if
the lookup fails.

trac-hacks/authopenid-plugin,trac-hacks/authopenid-plugin,trac-hacks/authopenid-plugin