'dfpre:' => 'match pre-image git blob ID',
'dfpost:' => 'match post-image git blob ID',
'dfblob:' => 'match either pre or post-image git blob ID',
+ 'patchid:' => "match `git patch-id --stable' output",
'rt:' => <<EOF,
match received time, like `d:' if sender's clock was correct
EOF