report_clear() efficiency plus clear after grep

2b384f6 introduced a performance regression due to use of wclear(),
which dirties the entire screen.  The core issue being addressed in
2b384f6 is that readline can leave an incorrect cursor position in
virtual window newscr.

So, explicitly reset the newscr cursor position post-readline, which
enables switching to the much more efficient werase() for erasing the
status area.

Incidentally fix the missed case of report_clear() after a grep prompt.

src/display.c

@@ -430,9 +430,8 @@ report_clear(void)
 		return;
 
 	if (!input_mode && !status_empty) {
-		wclear(status_win);
-		wclrtoeol(status_win);
-		wnoutrefresh(status_win);
+		werase(status_win);
+		doupdate();
 	}
 	status_empty = true;
 	update_view_title(view);

src/grep.c

@@ -99,6 +99,8 @@ grep_prompt(void)
 	int argc = 0;
 	char *grep = read_prompt("grep: ");
 
+	report_clear();
+
 	if (!grep || !argv_from_string_no_quotes(argv, &argc, grep))
 		return false;
 	if (grep_argv)

src/prompt.c

@@ -476,6 +476,9 @@ read_prompt(const char *prompt)
 	if (signal(SIGINT, SIG_DFL) == SIG_ERR)
 		die("Failed to remove sigint handler");
 
+	/* readline can leave the virtual cursor out-of-place */
+	set_cursor_pos(0, 0);
+
 	if (line && !*line) {
 		free(line);
 		line = NULL;