emacs: Take prompt and current tags in `notmuch-read-tag-changes'

emacs: Take prompt and current tags in `notmuch-read-tag-changes'

This modifies the interface of `notmuch-read-tag-changes' to take an
optional prompt string as well as a list of existing tags instead of a
query.  This list of tags is used to populate the tag removal
completions and lets the caller compute these in a more
efficient/consistent manner than performing a potentially large or
complex query.  This patch also updates the sole current caller of
`notmuch-read-tag-changes'.