Link & uniform formatting of SHAs in commit list

[timoxley]

This adds more consistency, plus links to commits are particularly useful.

FYI process was:

1. Click the io.js "new issue" button
2. Paste in the CHANGELOG markdown
3. Click "Preview". Note that plaintext commit SHAs in issues are automatically turned into HTML links.
4. Use devtools to grab generated HTML (now with commit links) from the preview
5. Drop HTML into an HTML -> Markdown editor: https://domchristie.github.io/to-markdown/
6. Copy/paste result into text editor
7. Use regexes to clean up formatting

[rvagg]

lgtm

landed @ 2e63bad with edited commit message