trace2:data: add editor/pager child classification

Add trace2 process classification for editor and pager
child processes.

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

editor.c

@@ -78,6 +78,7 @@ static int launch_specified_editor(const char *editor, const char *path,
 		p.argv = args;
 		p.env = env;
 		p.use_shell = 1;
+		p.trace2_child_class = "editor";
 		if (start_command(&p) < 0)
 			return error("unable to start editor '%s'", editor);