copying: when using M-6, copy the final line in the buffer just once

When using --nonewlines and the cursor has arrived at the end of the
final line, each press of M-6 would add another copy of that line to
the cutbuffer.  That is clearly not the right behavior.

Also, give feedback when a press of M-6 does not actually copy anything.

This fixes https://savannah.gnu.org/bugs/?58088.

The bug was old -- it existed since before version 2.2.0.

src/cut.c

@@ -576,6 +576,12 @@ void copy_text(void)
 		return;
 	}
 
+	if (openfile->current->next == NULL && at_eol && (ISSET(CUT_FROM_CURSOR) ||
+									openfile->current_x == 0 || cutbuffer)) {
+		statusbar(_("Copied nothing"));
+		return;
+	}
+
 	addition = make_new_node(NULL);
 
 	/* Create OR add to the cutbuffer, depending on the mode, the position