notmuch restore was skipping entries with no new tags

notmuch restore used to only add tags; now that it clears existing
tags, it needs to operate on messages even if the new tag list is empty.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Carl Worth <cworth@cworth.org>:
I fixed up the indentation here, (someday we might switch to 8-space
indents, but we haven't yet).