Summary line

- Imperitive mode: "Fix", "Add", "Change"
- Don't end with period, it's a title.
- Always leave the second line blank.

Message

- Why is it necessary? It may fix a bug, it may add a feature, it may
  improve performance, reliabilty, stability, or just be a change for
  the sake of correctness.

- How does it address the issue? For short obvious patches this part
  can be omitted, but it should be a high level description of what
  the approach was.

- What effects does the patch have? (In addition to the obvious ones,
  this may include benchmarks, side effects, etc.)

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Shamefully copied from a blog somewhere, all credits to the author!