emacs: Improve the acquisition of text parts.

`notmuch-get-bodypart-text' assumed that it is always possible to
acquire text/* parts via the sexp output format. This is not true if the
part in question has a content type of application/octet-stream but is
being interpreted as text/* based on the extension of the part filename.

Rework `notmuch-get-bodypart-text' to use the raw output format to
address this and make the implementation common with that of
`notmuch-get-bodypart-binary'.