i18n: Add function ngettext for pluralization.

* lisp/international/mule-cmds.el (ngettext): New function.
https://lists.gnu.org/archive/html/emacs-devel/2019-03/msg00586.html

* lisp/replace.el (flush-lines, how-many, occur-1, occur-engine)
(perform-replace): Use ngettext.

* lisp/progmodes/grep.el (grep-exit-message): Use ngettext.
(grep-mode-font-lock-keywords): Match both singular and plural form
of "matches".
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test-all

00:51:01