emacs: Fix misuse of `notmuch-tag'

emacs: Fix misuse of `notmuch-tag'

The calling convention for `notmuch-tag' changed in commit 97aa3c06 to
take a list of tag changes instead of a &rest argument, but the call
from `notmuch-search-tag-all' still passed a &rest argument.  This
happened to work for interactive calls because tag-changes would be
nil, so the `apply' call would pass only the query string to
`notmuch-tag' and simply omit the &optional tag-changes argument.