Fix off-by-one history pruning (bug#31211)

* lisp/subr.el (add-to-history): Clarify docstring.
Protect against negative history-length and unnecessary variable
modification, as per read_minibuf.

* lisp/ido.el (ido-record-command):
* lisp/international/mule-cmds.el (deactivate-input-method):
(set-language-environment-input-method):
* lisp/isearch.el (isearch-done):
* lisp/minibuffer.el (read-file-name-default):
* lisp/net/eww.el (eww-save-history):
* lisp/simple.el (edit-and-eval-command, repeat-complex-command):
(command-execute, kill-new, push-mark):
* src/callint.c (Fcall_interactively):
* src/minibuf.c (read_minibuf): Delegate to add-to-history.

* test/lisp/simple-tests.el (command-execute-prune-command-history):
* test/src/callint-tests.el
(call-interactively-prune-command-history): New tests.

lisp/subr.el

@@ -1798,7 +1798,7 @@ variable.  The possible values of maximum length have the same meaning as
 the values of `history-length'.
 Remove duplicates of NEWELT if `history-delete-duplicates' is non-nil.
 If optional fourth arg KEEP-ALL is non-nil, add NEWELT to history even
-if it is empty or a duplicate."
+if it is empty or duplicates the most recent entry in the history."
   (unless maxelt
     (setq maxelt (or (get history-var 'history-length)
 		     history-length)))
@@ -1814,12 +1814,12 @@ if it is empty or a duplicate."
 	  (setq history (delete newelt history)))
       (setq history (cons newelt history))
       (when (integerp maxelt)
-	(if (= 0 maxelt)
+        (if (>= 0 maxelt)
 	    (setq history nil)
 	  (setq tail (nthcdr (1- maxelt) history))
 	  (when (consp tail)
-	    (setcdr tail nil)))))
-    (set history-var history)))
+            (setcdr tail nil))))
+      (set history-var history))))
 
 
 ;;;; Mode hooks.