give Comment a timestamp, display in chronological order

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

src/bin/ent/main.rs

@@ -112,9 +112,10 @@ fn handle_command(args: &Args, issues_dir: &std::path::Path) -> anyhow::Result<(
                     }
                     println!("");
                     println!("{}", issue.description);
-                    for (uuid, comment) in issue.comments.iter() {
+                    for comment in &issue.comments {
                         println!("");
-                        println!("comment: {}", uuid);
+                        println!("comment: {}", comment.uuid);
+                        println!("timestamp: {}", comment.timestamp);
                         println!("{}", comment.description);
                     }
                 }