Fixes Nerdwin15/stash-jenkins-postreceive-webhook#78

![79](https://cloud.githubusercontent.com/assets/205553/4463515/9d2d9276-48cb-11e4-9114-56a661910504.png)

Hello, this allows omitting the SHA1 hash code of the commit. This is a workaround until https://issues.jenkins-ci.org/browse/JENKINS-24133 is fixed. Tested on Stash 2.9.4 and Stash 3.3.1.