contrib: pick: move save-excursion closer to message insertion

Pick keeps point roughly at the top of the buffer while inserting
messages at the end as they come in (from the async
parser). Previously the save-excursion to do this was done once for
each thread inserted: now it is done for each individual message.

The advantage is that the message insertion code can decide where to
leave point. In the next patch point will be left on the target message.

Note notmuch-pick-insert-msg is unchanged as that is used by the tag
display update code.