* lisp/progmodes/grep.el (grep-num-matches-found): New variable.

* lisp/progmodes/grep.el (grep-num-matches-found): New variable.

(grep-mode-line-matches): New defconst.
(grep-mode-font-lock-keywords): Update the regexp for “Grep finished”
to include the number of matches found.
(grep-process-setup): Set grep-num-matches-found to 0.
(grep-exit-message): New function with body moved from lambda
in grep-process-setup.  Use grep-num-matches-found to return
the number of matches found.
(grep-filter): Increment grep-num-matches-found.
(grep-mode): Set compilation-mode-line-errors to grep-mode-line-matches.
(Bug#30397, bug#14017)