prompt: suppress the ">" character always when exactly at the right edge

When the tail of the answer still fits exactly on the screen, the ">"
continuation character should not be shown -- also when the start of
the answer is "scrolled off" to the left.

This fixes https://savannah.gnu.org/bugs/?59816.

Bug existed in this form since version 4.0, commit 56181896.

src/prompt.c

@@ -403,7 +403,7 @@ void draw_the_promptbar(void)
 	waddstr(bottomwin, expanded);
 	free(expanded);
 
-	if (the_page < end_page && base + breadth(answer) != COLS)
+	if (the_page < end_page && base + breadth(answer) - the_page > COLS)
 		mvwaddch(bottomwin, 0, COLS - 1, '>');
 
 	wattroff(bottomwin, interface_color_pair[PROMPT_BAR]);