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.