Support for mirrorred repositories

When trying to scan a git repository checked out using `git clone --mirror`, the tool fails with an error:

```fatal: this operation must be run in a work tree```

To duplicate, try the following:
```
git clone --mirror https://github.com/awslabs/git-secrets.git
cd git-secrets.git/
git secrets --scan
```

Reproduce the failure by running `git clone --mirror` on the example repository, then `git secrets --scan` from the mirrored checkout. Trace the command entry point and how it invokes Git, and verify that scanning a mirror completes without the work-tree error while normal repositories still work.

Written by the indexing model from the issue text.