trace2:data: add trace2 sub-process classification

Add trace2 classification for long-running processes
started in sub-process.c

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sub-process.c

@@ -88,6 +88,7 @@ int subprocess_start(struct hashmap *hashmap, struct subprocess_entry *entry, co
 	process->out = -1;
 	process->clean_on_exit = 1;
 	process->clean_on_exit_handler = subprocess_exit_handler;
+	process->trace2_child_class = "subprocess";
 
 	err = start_command(process);
 	if (err) {