Can't pull master using Git Pull Endpoint and BitBucket

[nford]

This exception is always thrown when using the Git Pull Endpoint with BitBucket

https://github.com/aws-quickstart/quickstart-git2s3/blob/master/functions/source/GitPullS3/lambda_function.py#L193

End effect is that master is always pulled regardless of which branch triggered the BitBucket webhook.

[dewwwald]

+1

This is an important issue for us. Thank you @nford for your PR.