Compare case-folded nicks with erc-fill-wrap-merge

* lisp/erc/erc-fill.el (erc-fill--wrap-continued-message-p): Downcase
both current and previous speaker when comparing nicks.
* test/lisp/erc/erc-fill-tests.el (erc-fill-wrap--merge): Add
title-case nick to test.
* test/lisp/erc/resources/fill/snapshots/merge-01-start.eld: Update
snapshot.
* test/lisp/erc/resources/fill/snapshots/merge-02-right.eld: Update
snapshot.  (Bug#60936)

lisp/erc/erc-fill.el

@@ -363,7 +363,8 @@ parties.")
                             erc-fill--wrap-max-lull))
               (nick  (buffer-substring-no-properties
                       (1+ (point-min)) (- (point) 2)))
-              ((equal (car props) (erc-downcase nick)))))
+              (props)
+              ((erc-nick-equal-p (car props) nick))))
     (set-marker erc-fill--wrap-last-msg (point-min))))
 
 (defun erc-fill--wrap-stamp-insert-prefixed-date (args)

test/lisp/erc/erc-fill-tests.el

@@ -203,18 +203,23 @@
   (erc-fill-tests--wrap-populate
 
    (lambda ()
+     (erc-update-channel-member
+      "#chan" "Dummy" "Dummy" t nil nil nil nil nil "fake" "~u" nil nil t)
+
      ;; Set this here so that the first few messages are from 1970
      (let ((erc-fill-tests--time-vals (lambda () 1680332400)))
        (erc-fill-tests--insert-privmsg "bob" "zero.")
        (erc-fill-tests--insert-privmsg "alice" "one.")
        (erc-fill-tests--insert-privmsg "alice" "two.")
        (erc-fill-tests--insert-privmsg "bob" "three.")
-       (erc-fill-tests--insert-privmsg "bob" "four."))
+       (erc-fill-tests--insert-privmsg "bob" "four.")
+       (erc-fill-tests--insert-privmsg "Dummy" "five.")
+       (erc-fill-tests--insert-privmsg "Dummy" "six."))
 
      (should (= erc-fill--wrap-value 27))
      (erc-fill-tests--wrap-check-prefixes
       "*** " "<alice> " "<bob> "
-      "<bob> " "<alice> " "<alice> " "<bob> " "<bob> ")
+      "<bob> " "<alice> " "<alice> " "<bob> " "<bob> " "<Dummy> " "<Dummy> ")
      (erc-fill-tests--compare "merge-01-start")
 
      (ert-info ("Shift right by one (plus)")
@@ -222,7 +227,7 @@
        (should (= erc-fill--wrap-value 29))
        (erc-fill-tests--wrap-check-prefixes
         "*** " "<alice> " "<bob> "
-        "<bob> " "<alice> " "<alice> " "<bob> " "<bob> ")
+        "<bob> " "<alice> " "<alice> " "<bob> " "<bob> " "<Dummy> " "<Dummy> ")
        (erc-fill-tests--compare "merge-02-right")))))
 
 (ert-deftest erc-fill-wrap-visual-keys--body ()