From http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html:

• 1st line: Capitalized, short (50 chars or less) summary
• Wrap more comments at 72 columns
• Use – or * as a bullet, left justify

From https://github.com/torvalds/linux/pull/17#issuecomment-5654674:

• Pull requests should have a signed tag