emacs: Avoid runtime use of `cl'.

emacs: Avoid runtime use of `cl'.

The GNU Emacs Lisp Reference Manual section D.1 says:

> *  Please don't require the cl package of Common Lisp extensions at
>    run time. Use of this package is optional, and it is not part of
>    the standard Emacs namespace. If your package loads cl at run time,
>    that could cause name clashes for users who don't use that package.
>
>    However, there is no problem with using the cl package at compile
>    time, with (eval-when-compile (require 'cl)). That's sufficient for
>    using the macros in the cl package, because the compiler expands
>    them before generating the byte-code.

Follow this advice, requiring the following changes where `cl' was
used at runtime:

- replace `rassoc-if' in `notmuch-search-buffer-title' with the `loop'
  macro and inline code. At the same time find the longest prefix
  which matches the query rather than simply the last,
- replace `union', `intersection' and `set-difference' in
  `notmuch-show-add-tag' and `notmuch-show-remove-tag' with local code
  to calculate the result of adding and removing a list of tags from
  another list of tags.