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.
http://example.com/no/anchor, branches: []
http://example.com/an/anchor, branches: ['foo']
http://example.com/no/anchor/branches, branches: ['foo']
http://example.com/an/anchor/branches, branches: ['foo', 'bar']
http://example.com/an/anchor/branches-None, branches: ['foo']