Make comint-read-input-ring skip uninteresting text in .zsh_history

Make comint-read-input-ring skip uninteresting text in .zsh_history

* lisp/comint.el (comint-read-input-ring): Simplify (bug#45606).
* lisp/shell.el (shell-mode): Add "~/.zsh_history".
* lisp/comint.el (comint-read-input-ring): Bind
`comint-input-ring-file-prefix' in anticipation of a buffer switch.
* lisp/comint.el (comint-read-input-ring): Skip the separator.

Because re-search-backward moves point to the beginning of the match,
and since we don't want the separator appearing in the output, we skip
over it.

This is required to properly detect instances of the value that zsh
uses for `comint-input-ring-file-prefix'; if the
`comint-input-ring-file-prefix' is ':potato', the subsequent
invocation `looking-at' sees '\n:potato' for all entries after the one
at the very beginning of the history file.