message: catch wrong return types
diff: @@ -135,13 +135,15 @@ describe('git merge', function() {
  let updatedContent = [
  { operation: 'create', filename: 'hello-world.txt', buffer: Buffer.from('This is a file', 'utf8') }
  ];
  - await git.mergeCommit(repo, parentRef.target(), 'master', updatedContent, commitOpts({ message: 'Second commit' }));
  + let commitId = await git.mergeCommit(repo, parentRef.target(), 'master', updatedContent, commitOpts({ message: 'Second commit' }));
  + expect(commitId).is.a('string');
  updatedContent = [
  { operation: 'create', filename: 'other.txt', buffer: Buffer.from('Non-conflicting content', 'utf8') }
  ];
  // This is based on the same parentRef as the second commit, so it's not a fast forward
  - await git.mergeCommit(repo, parentRef.target(), 'master', updatedContent, commitOpts({ message: 'Third commit' }));
  + commitId = await git.mergeCommit(repo, parentRef.target(), 'master', updatedContent, commitOpts({ message: 'Third commit' }));
  + expect(commitId).is.a('string');
  expect((await inRepo(path).getCommit('master')).message).to.equal('Clean merge into master');
  expect((await inRepo(path).getCommit('master^1')).message).to.equal('Third commit');
  