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

src/lib/get-commits.js

@@ -0,0 +1,56 @@
+const execa = require('execa');
+const log = require('npmlog');
+const SemanticReleaseError = require('@semantic-release/error');
+
+module.exports = async ({lastRelease, options}) => {
+  let stdout;
+  if (lastRelease.gitHead) {
+    try {
+      ({stdout} = await execa('git', ['branch', '--no-color', '--contains', lastRelease.gitHead]));
+    } catch (err) {
+      throw notInHistoryError(lastRelease.gitHead, options.branch);
+    }
+    const branches = stdout
+      .split('\n')
+      .map(branch => branch.replace('*', '').trim())
+      .filter(branch => !!branch);
+
+    if (!branches.includes(options.branch)) {
+      throw notInHistoryError(lastRelease.gitHead, options.branch, branches);
+    }
+  }
+
+  try {
+    ({stdout} = await execa('git', [
+      'log',
+      '--format=%H==SPLIT==%B==END==',
+      `${lastRelease.gitHead ? lastRelease.gitHead + '..' : ''}HEAD`,
+    ]));
+  } catch (err) {
+    return [];
+  }
+
+  return String(stdout)
+    .split('==END==')
+    .filter(raw => !!raw.trim())
+    .map(raw => {
+      const [hash, message] = raw.trim().split('==SPLIT==');
+      return {hash, message};
+    });
+};
+
+function notInHistoryError(gitHead, branch, branches) {
+  log.error(
+    'commits',
+    `
+The commit the last release of this package was derived from is not in the direct history of the "${branch}" branch.
+This means semantic-release can not extract the commits between now and then.
+This is usually caused by force pushing, releasing from an unrelated branch, or using an already existing package name.
+You can recover from this error by publishing manually or restoring the commit "${gitHead}".
+${branches && branches.length
+      ? `\nHere is a list of branches that still contain the commit in question: \n * ${branches.join('\n * ')}`
+      : ''}
+`
+  );
+  return new SemanticReleaseError('Commit not in history', 'ENOTINHISTORY');
+}