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.
adding a/b/c/d/x
adding a/b/c/d/y
adding a/b/c/d/z
moving a/b/c/d/x to z/b/c/d/x
moving a/b/c/d/y to z/b/c/d/y
moving a/b/c/d/z to z/b/c/d/z