Remove duplicate ERC prompt on reconnect

* lisp/erc/erc-backend.el (erc--unhide-prompt, erc--hide-prompt,
erc--unhide-prompt-on-self-insert): Add functions to ensure prompt is
hidden on disconnect and shown when a user types /reconnect in a
disconnected server buffer.
(erc-process-sentinel): Register aforementioned function with
`pre-command-hook' when prompt is deleted after disconnecting.
(erc-server-PRIVMSG): Ensure prompt is showing when a new message
arrives from target.

* lisp/erc/erc.el (erc-hide-prompt): Repurpose unused option by
changing meaning slightly to mean "selectively hide prompt when
disconnected."  Also delete obsolete, commented-out code that at some
point used this option in its prior incarnation.
(erc-prompt-hidden): Add new option to specify look of prompt when
hidden.
(erc-unhide-query-prompt): Add option to force-reveal query prompts on
reconnect.
(erc-open): Augment earlier reconnect-detection semantics by
incorporating `erc--server-reconnecting'.  In existing buffers, remove
prompt-related hooks and reveal prompt, if necessary.
(erc-cmd-RECONNECT): Allow a user to reconnect when already
connected (by first disconnecting).
(erc-connection-established): Possibly unhide query prompts.
(Bug#54826)

* test/lisp/erc/erc-tests.el (erc-tests--test-prep,
erc-tests--set-fake-server-process): Factor out some common
buffer-prep boilerplate involving user input and the server process.
Shared with bug#54536.

lisp/erc/erc-backend.el

@@ -705,6 +705,39 @@ Conditionally try to reconnect and take appropriate action."
       ;; unexpected disconnect
       (erc-process-sentinel-2 event buffer))))
 
+(defun erc--unhide-prompt ()
+  (remove-hook 'pre-command-hook #'erc--unhide-prompt-on-self-insert t)
+  (when (and (marker-position erc-insert-marker)
+             (marker-position erc-input-marker))
+    (with-silent-modifications
+      (remove-text-properties erc-insert-marker erc-input-marker
+                              '(display nil)))))
+
+(defun erc--unhide-prompt-on-self-insert ()
+  (when (and (eq this-command #'self-insert-command)
+             (or (eobp) (= (point) erc-input-marker)))
+    (erc--unhide-prompt)))
+
+(defun erc--hide-prompt (proc)
+  (erc-with-all-buffers-of-server
+      proc nil ; sorta wish this was indent 2
+      (when (and erc-hide-prompt
+                 (or (eq erc-hide-prompt t)
+                     ;; FIXME use `erc--target' after bug#48598
+                     (memq (if (erc-default-target)
+                               (if (erc-channel-p (car erc-default-recipients))
+                                   'channel
+                                 'query)
+                             'server)
+                           erc-hide-prompt))
+                 (marker-position erc-insert-marker)
+                 (marker-position erc-input-marker)
+                 (get-text-property erc-insert-marker 'erc-prompt))
+        (with-silent-modifications
+          (add-text-properties erc-insert-marker (1- erc-input-marker)
+                               `(display ,erc-prompt-hidden)))
+        (add-hook 'pre-command-hook #'erc--unhide-prompt-on-self-insert 0 t))))
+
 (defun erc-process-sentinel (cproc event)
   "Sentinel function for ERC process."
   (let ((buf (process-buffer cproc)))
@@ -727,11 +760,8 @@ Conditionally try to reconnect and take appropriate action."
           (dolist (buf (erc-buffer-filter (lambda () (boundp 'erc-channel-users)) cproc))
             (with-current-buffer buf
               (setq erc-channel-users (make-hash-table :test 'equal))))
-          ;; Remove the prompt
-          (goto-char (or (marker-position erc-input-marker) (point-max)))
-          (forward-line 0)
-          (erc-remove-text-properties-region (point) (point-max))
-          (delete-region (point) (point-max))
+          ;; Hide the prompt
+          (erc--hide-prompt cproc)
           ;; Decide what to do with the buffer
           ;; Restart if disconnected
           (erc-process-sentinel-1 event buf))))))
@@ -1479,6 +1509,7 @@ add things to `%s' instead."
         (setq buffer (erc-get-buffer (if privp nick tgt) proc))
         (when buffer
           (with-current-buffer buffer
+            (when privp (erc--unhide-prompt))
             ;; update the chat partner info.  Add to the list if private
             ;; message.  We will accumulate private identities indefinitely
             ;; at this point.