display: clear the remainder of a row only when there actually is some

Make display_string() report back (via a global variable) whether the
just converted string has room to spare within the allotted span, so
that draw_row() can avoid trying to clear the remainder when there is
nothing to clear.

This fixes https://savannah.gnu.org/bugs/?56562.

Bug existed since commit 699cacf7 from three days ago.

src/global.c

@@ -68,6 +68,9 @@ bool more_than_one = FALSE;
 bool also_the_last = FALSE;
 		/* Whether indenting/commenting should include the last line of
 		 * the marked region. */
+bool is_shorter;
+		/* Whether a row's text is narrower than the screen's width. */
+
 int didfind = 0;
 		/* Whether the last search found something. */
 

src/proto.h

@@ -55,6 +55,8 @@ extern bool more_than_one;
 
 extern bool also_the_last;
 
+extern bool is_shorter;
+
 extern int didfind;
 
 extern int controlleft, controlright;

src/winio.c

@@ -2023,6 +2023,8 @@ char *display_string(const char *buf, size_t column, size_t span,
 #endif
 	}
 
+	is_shorter = (column < beyond);
+
 	/* Null-terminate the converted string. */
 	converted[index] = '\0';
 
@@ -2440,7 +2442,10 @@ void draw_row(int row, const char *converted, linestruct *line, size_t from_col)
 	/* First simply write the converted line -- afterward we'll add colors
 	 * and the marking highlight on just the pieces that need it. */
 	mvwaddstr(edit, row, margin, converted);
-	wclrtoeol(edit);
+
+	/* When needed, clear the remainder of the row. */
+	if (is_shorter || ISSET(SOFTWRAP))
+		wclrtoeol(edit);
 
 #ifdef USING_OLD_NCURSES
 	/* Tell ncurses to really redraw the line without trying to optimize