nmbug: use 'notmuch tag --batch'

nmbug: use 'notmuch tag --batch'

This should be more robust with respect to tags with whitespace and
and other special characters. It also (hopefully) fixes a remaining
bug handling message-ids with whitespace.  It should also be
noticeably faster for large sets of changes since it does one exec per
change set as opposed to one exec per tag changed.