cli/reply: pass gmime message to Reply-To: redundancy detection

Use gmime message instead of notmuch message in Reply-To: redundancy
detection. This allows us to easily iterate over all recipient email
addresses accurately, instead of just scanning for strings in the
relevant message headers. This improves the accuracy of the detection
in many ways.

This also makes the notmuch message parameter to get_sender()
unused. This will be cleaned up in a follow-up patch to not make too
many changes here at once.