display: highlight the ">"/"<" continuation characters in reverse video

To make it obvious that they are not actual characters in the file.

The default highlighting is in reverse video, but this can be changed
to bold by using --bold or 'set boldtext'.

This fulfills https://savannah.gnu.org/bugs/?55571.

src/winio.c

@@ -2776,10 +2776,16 @@ int update_line(filestruct *fileptr, size_t index)
 	edit_draw(fileptr, converted, row, from_col);
 	free(converted);
 
-	if (from_col > 0)
+	if (from_col > 0) {
+		wattron(edit, hilite_attribute);
 		mvwaddch(edit, row, margin, '<');
-	if (strlenpt(fileptr->data) > from_col + editwincols)
+		wattroff(edit, hilite_attribute);
+	}
+	if (strlenpt(fileptr->data) > from_col + editwincols) {
+		wattron(edit, hilite_attribute);
 		mvwaddch(edit, row, COLS - 1, '>');
+		wattroff(edit, hilite_attribute);
+	}
 
 	if (spotlighted && !inhelp)
 		spotlight(light_from_col, light_to_col);