refactor: Use ES6, Test with AVA

- Use async/await instead of callbacks
- Use execa to run command line
- Use AVA for tests
- Add several assertions in the unit tests
- Add documentation (comments) in the tests
- Run tests with a real git repo instead of mocking child_process and add test helpers to create repos, commits and checkout
- Simplify test directory structure
- Simplify code readability (mostly with async/await)
- Use eslint for for linting, prettier for formatting

bin/semantic-release.js

@@ -3,11 +3,11 @@
 // Bad news: We have to write plain ES5 in this file
 // Good news: It's the only file of the entire project
 
-var semver = require('semver')
+var semver = require('semver');
 
 if (semver.lt(process.version, '8.0.0')) {
   console.error(
-`semantic-release: node version >= 8 is required. Found ${process.version}.
+    `semantic-release: node version >= 8 is required. Found ${process.version}.
 
 If there is another job running on node version >= 8, it will be picked as
 the build leader and you can safely ignore this message.
@@ -17,9 +17,10 @@ compatibility with minimal overhead:
 
 $ npx -p node@8 npm run semantic-release
 
-npx is bundled with npm >= 5.4, or available via npm. More info: npm.im/npx`)
-  process.exit(1)
+npx is bundled with npm >= 5.4, or available via npm. More info: npm.im/npx`
+  );
+  process.exit(1);
 }
 
 // node 8+ from this point on
-require('../src')
+require('../src')();