notmuch part: Fix part numbering to match what's reported by "notmuch show"

Since commit c51d5b3cdb5ca0816816e88ca6f7136a24e74eee we are counting
multipart containers when emitting part numbers in the "notmuch show"
output. Unfortunately, "notmuch part" wasn't updated with the same
numbering, (and the test suite is inadequate to catch this).

Fix this by adding a similar part-numbering change to "notmuch part" here.