notmuch-emacs-mua: escape $PWD (and cd always)

notmuch-emacs-mua: escape $PWD (and cd always)

Escaping $PWD makes this work in directories like 'foo"bar'...

Cd'ing always makes the working directory to be consistent whether
--body option was used or not (when using emacsclient, but cd'ing
when using emacs does not cause any harm).

Note that documentation of `insert-file` expects programs to
call `insert-file-contents` instead. In our simple case
`insert-file` works better as it does some good checks that we'd
have to implement ourselves. Look lisp/files.el in emacs sources
for more information.