CLI/show: support --duplicate for structured output
This introduces a new mandatory key for message structures, namely
"duplicate". Per convention in devel/schemata this does _not_ increase
the format version. This means that clients are responsible for
checking that it exists, and not crashing if it does not.
The main functional change is teaching mime_node_open to understand a
'duplicate' argument.
Support for --duplicate in notmuch-reply would make sense, but we
defer it to a later commit.
projects / notmuch / commit