Rewrite gnus-search-query-expand-key

* lisp/gnus/gnus-search.el (gnus-search-query-expand-key): There was a
misunderstanding about how completion-all-completion works (if the
test string can't be completed, the whole table is returned). Simplify
to use try-completion.
* test/lisp/gnus/gnus-search-tests.el (gnus-s-expand-keyword): Ensure
that an unknown/uncompletable keyword is returned unmolested.

lisp/gnus/gnus-search.el

@@ -629,18 +629,16 @@ gnus-*-mark marks, and return an appropriate string."
     mark))
 
 (defun gnus-search-query-expand-key (key)
-  (cond ((test-completion key gnus-search-expandable-keys)
-	 ;; We're done!
-	 key)
-	;; There is more than one possible completion.
-	((consp (cdr (completion-all-completions
-		      key gnus-search-expandable-keys #'stringp 0)))
-	 (signal 'gnus-search-parse-error
-		 (list (format "Ambiguous keyword: %s" key))))
-	;; Return KEY, either completed or untouched.
-	((car-safe (completion-try-completion
-		    key gnus-search-expandable-keys
-		    #'stringp 0)))))
+  (let ((comp (try-completion key gnus-search-expandable-keys)))
+    (if (or (eql comp 't)		; Already a key.
+	    (null comp))		; An unknown key.
+	key
+      (if (string= comp key)
+	  ;; KEY matches multiple possible keys.
+	  (signal 'gnus-search-parse-error
+		  (list (format "Ambiguous keyword: %s" key)))
+	;; We completed to a unique known key.
+	comp))))
 
 (defun gnus-search-query-return-string (&optional delimited trim)
   "Return a string from the current buffer.

test/lisp/gnus/gnus-search-tests.el

@@ -49,7 +49,9 @@
          (default-value 'gnus-search-expandable-keys))
         (pairs
          '(("su" . "subject")
-           ("sin" . "since"))))
+           ("sin" . "since")
+           ("body" . "body")
+           ("list-id" . "list-id"))))
     (dolist (p pairs)
       (should (equal (gnus-search-query-expand-key (car p))
                      (cdr p))))