message: add system props test
diff: import React from 'react'
  import BranchName from '../BranchName'
  import {render, rendersClass} from '../utils/testing'
  +import {COMMON} from '../system-props'
  describe('BranchName', () => {
  it('renders an <a> by default', () => {
  @@ -26,4 +27,7 @@ describe('BranchName', () => {
  xit('respects padding utility prop', () => {
  expect(rendersClass(<BranchName p={1} />, 'p-1')).toEqual(true)
  })
  + it('implements common system props', () => {
  + expect(BranchName).toImplementSystemProps(COMMON)
  + })
  })
  