cross-ci/lib/var/BUILD_COMMIT7.js

`cross-ci` standardizes environment variables for CI. For example, your can simply use `BUILD_BRANCH` variable in all CI runners instead of `CIRCLE_BRANCH` in CircleCI or `TRAVIS_PULL_REQUEST_BRANCH` in Travis.

const BUILD_COMMIT = require('./BUILD_COMMIT');

/// First 7 chars of SHA1 of the Git commit being built.
const BUILD_COMMIT7 = () =>
    (BUILD_COMMIT() || '').slice(0, 7);

module.exports = BUILD_COMMIT7;