cli: refactor "notmuch tag" data structures for tagging operations

cli: refactor "notmuch tag" data structures for tagging operations

To simplify code, keep all tagging operations in a single array
instead of separate add and remove arrays. Apply tag changes in the
order specified on the command line, instead of first removing and
then adding the tags.

This results in a minor functional change: If a tag is both added and
removed, the last specified operation is now used. Previously the tag
was always added. Change the relevant test to reflect the new
behaviour.

Signed-off-by: Jani Nikula <jani@nikula.org>