emacs: Use S-exp format everywhere

This switches `notmuch-mua-reply' and `notmuch-query-get-threads' to
the S-exp format.  These were the last two uses of the JSON format in
the Emacs frontend.

emacs/notmuch-mua.el

@@ -146,7 +146,7 @@ list."
   (unless (bolp) (insert "\n")))
 
 (defun notmuch-mua-reply (query-string &optional sender reply-all)
-  (let ((args '("reply" "--format=json" "--format-version=1"))
+  (let ((args '("reply" "--format=sexp" "--format-version=1"))
 	reply
 	original)
     (when notmuch-show-process-crypto
@@ -158,7 +158,7 @@ list."
     (setq args (append args (list query-string)))
 
     ;; Get the reply object as JSON, and parse it into an elisp object.
-    (setq reply (apply #'notmuch-call-notmuch-json args))
+    (setq reply (apply #'notmuch-call-notmuch-sexp args))
 
     ;; Extract the original message to simplify the following code.
     (setq original (plist-get reply :original))

emacs/notmuch-query.el

@@ -29,11 +29,11 @@ A thread is a forest or list of trees. A tree is a two element
 list where the first element is a message, and the second element
 is a possibly empty forest of replies.
 "
-  (let ((args '("show" "--format=json" "--format-version=1")))
+  (let ((args '("show" "--format=sexp" "--format-version=1")))
     (if notmuch-show-process-crypto
 	(setq args (append args '("--decrypt"))))
     (setq args (append args search-terms))
-    (apply #'notmuch-call-notmuch-json args)))
+    (apply #'notmuch-call-notmuch-sexp args)))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Mapping functions across collections of messages.

test/emacs-show

@@ -189,7 +189,7 @@ This is an error (see *Notmuch errors* for more details)
 ---
 [XXX]
 This is an error
-command: $PWD/notmuch_fail show --format\\=json --format-version\\=1 --exclude\\=false \\' \\* \\'
+command: $PWD/notmuch_fail show --format\\=sexp --format-version\\=1 --exclude\\=false \\' \\* \\'
 exit status: 1
 stderr:
 This is an error