Link & uniform formatting of SHAs in commit list
This adds more consistency, plus links to commits are particularly useful.
FYI process was:
- Click the io.js "new issue" button
- Paste in the CHANGELOG markdown
- Click "Preview". Note that plaintext commit SHAs in issues are automatically turned into HTML links.
- Use devtools to grab generated HTML (now with commit links) from the preview
- Drop HTML into an HTML -> Markdown editor: https://domchristie.github.io/to-markdown/
- Copy/paste result into text editor
- Use regexes to clean up formatting