Add a $status_generation variable that's incremented for each interactive command that produces a status.

This can be used to determine whether the previous command produced a real status, or just carried over the status from the command before it. Backgrounded commands and variable assignments will not increment status_generation, all other commands will.

src/parse_execution.cpp

@@ -393,6 +393,7 @@ end_execution_reason_t parse_execution_context_t::run_function_statement(
     buffered_output_stream_t errs(0);
     io_streams_t streams(outs, errs);
     int err = builtin_function(*parser, streams, arguments, pstree, statement);
+    parser->libdata().status_count++;
     parser->set_last_statuses(statuses_t::just(err));
 
     wcstring errtext = errs.contents();