[#6111][#6140] Improve commit message rendering

- Handle Trac-style artifact refs
- Only do minimal Markdown conversion (refs and linebreaks)

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
[#6140] Minor tweak to source-style trac links

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>