It's called the Imperative Mood[0]. I like to think of it as though I'm completing this statement: "If you apply this patch, this commit will [Do The Thing™]". Here's a few examples:

If I were to apply this patch, this commit will…

- Simplify FooWidget

- Add optional table support

- Remove trailing whitespace

- Update fizzbuzz to 1.2.0

- Merge branch 'feature/ABC-123-foobar'

- Revert "Add foo to bar"

[0]: https://en.wikipedia.org/wiki/Imperative_mood