The alphabetical ordering of keys in ssh-agent affects the order in
which keys are offered to remotes. MaxAuthTries is an sshd setting that
will raise a "Too many authentication failures" error after it has been
offered more than MaxAuthTries keys.
The function get_keyholder_key seems to do the right thing in all
deployed repos including MediaWiki. I added the additional configuration
variable mediawiki_keyholder_key to allow us to set the key for scap2
deployments without having scap3 repos inherit the configuration.