tags: return tags in sorted order
This makes log and summary have less arbitrary tag ordering and also
groups similar tags together, such as the mq tags.
% fail
abort: inotify-server: cannot start: .hg/inotify.sock is a broken symlink
inotify-client: could not start inotify server: child process failed to start
abort: inotify-server: cannot start: .hg/inotify.sock is a broken symlink
% inserve
% status
? hg.pid