comint-insert-previous-argument doesn't detect and ignore trailing &

This function is invoked in shell-mode by the user, and is meant to
emulate what M-. does in zsh and bash: it inserts an argument from a
previous command.  Neither zsh nor bash treat a trailing & specially:
M-. simply inserts it if it is encountered.  Emacs DID have extra
logic to detect and discard trailing &, but this logic was buggy, and
a && anywhere in the sequence would confuse it.  This patch simply
removes that logic to fix the bug and to emulate zsh and bash more
closely

* lisp/comint.el (comint-insert-previous-argument): don't detect and
  ignore trailing &
  (Bug#25271)
* etc/NEWS: Document this.

etc/NEWS

@@ -191,6 +191,11 @@ It now treats the optional 2nd argument to mean that the URL should be
 shown in the currently selected window.
 
 ** Comint
+*** 'comint-insert-previous-argument' no longer interprets &.
+This worked strangely in shell-mode in the presence of &&.  And omitting this
+logic makes sense since 'comint-insert-previous-argument' exists to emulate M-.
+in bash and zsh, and neither of those treat & specially.
+
 *** 'comint-insert-previous-argument' knows how to count args
 from the beginning or from the end.  This is useful because
 'comint-insert-previous-argument' exists to emulate M-. in bash and zsh; and

lisp/comint.el

@@ -2705,9 +2705,6 @@ is the last argument.  This command is like `M-.' in bash and zsh."
   (set-marker comint-insert-previous-argument-last-start-pos (point))
   ;; Insert the argument.
   (let ((input-string (comint-previous-input-string 0)))
-    (when (string-match "[ \t\n]*&" input-string)
-      ;; strip terminating '&'
-      (setq input-string (substring input-string 0 (match-beginning 0))))
     (insert (comint-arguments input-string index index)))
   ;; Make next invocation return arg from previous input
   (setq comint-input-ring-index (1+ (or comint-input-ring-index 0)))