This commit makes a couple of changes:
- `ent show ISSUE` now displays the Issue's Comments in chronological
order
- the Comment struct now includes a timestamp, which is the Author Time
of the oldest commit that touches the comment's directory
- the Issue struct now stores its Comments in a sorted Vec, not in
a HashMap
- The Comment's uuid moved into the Comment struct itself, instead of
being the key in the Issue's HashMap of Comments
