bug#34520: delete-matching-lines should report how many lines it deleted

[Juri Linkov]

> Got an idea!: Simply keep a count of how many lines were deleted, and
> report that in the minibuffer, if using interactively.

Good idea, this will bring deleting the lines under control:

diff --git a/lisp/replace.el b/lisp/replace.el
index b482d76afc..c3d8278936 100644
--- a/lisp/replace.el
+++ b/lisp/replace.el
@@ -953,7 +953,10 @@ flush-lines
 
 If a match is split across lines, all the lines it lies in are deleted.
 They are deleted _before_ looking for the next match.  Hence, a match
-starting on the same line at which another match ended is ignored."
+starting on the same line at which another match ended is ignored.
+
+Return the number of deleted lines.  When called interactively,
+also print the number."
   (interactive
    (progn
      (barf-if-buffer-read-only)
@@ -968,7 +971,8 @@ flush-lines
       (setq rstart (point)
            rend (point-max-marker)))
     (goto-char rstart))
-  (let ((case-fold-search
+  (let ((count 0)
+        (case-fold-search
         (if (and case-fold-search search-upper-case)
             (isearch-no-upper-case-p regexp t)
           case-fold-search)))
@@ -978,9 +982,13 @@ flush-lines
        (delete-region (save-excursion (goto-char (match-beginning 0))
                                       (forward-line 0)
                                       (point))
-                      (progn (forward-line 1) (point))))))
-  (set-marker rend nil)
-  nil)
+                      (progn (forward-line 1) (point)))
+        (setq count (1+ count))))
+    (set-marker rend nil)
+    (when interactive (message "Deleted %d matching line%s"
+                              count
+                              (if (= count 1) "" "s")))
+    count))
 
 
 (defun how-many (regexp &optional rstart rend interactive)