type: 'git-commit' doesn't work from git submodule

Related to https://github.com/rwjblue/git-repo-info/pull/11

From a git submodule I get the following error

``` sh
+- 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/ewan/projects/questionr/main/node_modules/ember-cli-deploy-revision-data/lib/data-generators/git-commit.js:14:23)
```

As a workaround in deploy.js I'm using

``` js
var exec = require('child_process').execSync;

....

redis: {
revisionKey: function(){
return exec('git rev-parse --short --verify HEAD').toString().replace('\n','');
}
}
```

Start by reading node_modules/ember-cli-deploy-revision-data/lib/data-generators/git-commit.js around line 14 and compare its git metadata handling with the reported git rev-parse --short --verify HEAD workaround. Done means the git-commit revision-data generator works when run from a git submodule without the TypeError.

Written by the indexing model from the issue text.