Support git bundle

When the copy destination is a local path and ends with `.git-bundle`, we can bundle the repo into single file, like

```
git copy http://github.com/cybertk/git-copy.git /path/to/git-copy.git-bundle
```

Then restore with

```
git clone /path/to/git-copy.git-bundle /my/workspace
```

This is inspired by http://stackoverflow.com/a/5578292/622662