Suppress git error output when site isn't under source control.

Hi gjtorikian, Nice plugin. Here's a patch for us poor souls laboring outside of GIT repos. - chuckhoupt

#### Details:

When working in a directory that isn't part of a GIT repo, the plugin generates the following error message for each file processed:

```
fatal: Not a git repository (or any of the parent directories): .git
```

This patch fixes this by dev-nulling the output from the 'git rev-parse' command in is_git_repo?()