notmuch-search-get-tags unique candidates

[Nicolò Balzarotti]

Hi, I'm using notmuch with emacs and I'm loving it. One thing I don't
like is that in `notmuch-search`, when applying a tag to a selection the
tab completition return multiple copies of the same tag. Example:

> -inbox
> -lists/aaa
> -lists
> -inbox
> -lists/aaa
> -lists

I changed `notmuch-search-get-tags-region` to:
```(defun notmuch-search-get-tags-region (beg end)
  (let (output)
    (notmuch-search-foreach-result beg end
      (lambda (pos)
	(setq output (append output (notmuch-search-get-tags pos)))))
    (delete-dups output)));; add delete-dups before returning
    ```

Why is the current one the default behavior? 

Thanks, Nicolò