Deployment fails to prepare when using 'git-commit'

When using scm-data-generator 'git-commit' in CirlcleCI, prepare fails.

```
+- prepare
| |
| +- revision-data
| - creating revision data using `git-commit`
|
+- didFail
TypeError: Cannot read property 'slice' of null
TypeError: Cannot read property 'slice' of null
at CoreObject.module.exports.CoreObject.extend.generate (/home/ubuntu/the-order/node_modules/ember-cli-deploy-revision-data/lib/data-generators/git-commit.js:14:23)
at Class.DeployPluginBase.extend._getData (/home/ubuntu/the-order/node_modules/ember-cli-deploy-revision-data/index.js:59:12)
at Class.DeployPluginBase.extend.prepare (/home/ubuntu/the-order/node_modules/ember-cli-deploy-revision-data/index.js:36:24)
at Object._pipeline.register.fn (/home/ubuntu/the-order/node_modules/ember-cli-deploy/lib/tasks/pipeline.js:144:21)
at Pipeline._notifyPipelinePluginHookExecution (/home/ubuntu/the-order/node_modules/ember-cli-deploy/lib/models/pipeline.js:165:19)
at lib$rsvp$$internal$$tryCatch (/home/ubuntu/the-order/node_modules/rsvp/dist/rsvp.js:1036:16)
at lib$rsvp$$internal$$invokeCallback (/home/ubuntu/the-order/node_modules/rsvp/dist/rsvp.js:1048:17)
at /home/ubuntu/the-order/node_modules/rsvp/dist/rsvp.js:331:11
at lib$rsvp$asap$$flush (/home/ubuntu/the-order/node_modules/rsvp/dist/rsvp.js:1198:9)
at doNTCallback0 (node.js:430:9)|
Pipeline aborted
```

Per @ghedamat the fix will likely need to happen in

https://github.com/ember-cli-deploy/ember-cli-deploy-revision-data/blob/master/lib/data-generators/git-commit.js

and also need to path these two lines

https://github.com/ember-cli-deploy/ember-cli-deploy-revision-data/blob/master/lib/scm-data-generators/git.js#L17-L18