For faster sync, add option to only download a single branch

**Describe the problem/challenge you have**
Currently, `vendir` takes long time to clone a repository. If I am after files from a specific branch it takes long time and it downloads unnecessary stuff I don't care about.

**Describe the solution you'd like**

Add an option to
- specify cloning depth
- only clone single branch

```yml
apiVersion: vendir.k14s.io/v1alpha1
kind: Config
directories:
- path: docs/modules
contents:
- path: ROOT/partials
git:
url: https://github.com/openshift/openshift-docs
ref: rhacs-docs
single-branch: true
depth: 1
includePaths:
- modules/*
newRootPath: modules/
```

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No implementation file or test is named. Start by tracing how vendir reads the YAML git source shown in the example and performs repository cloning; done means the configuration supports both depth and single-branch behavior while preserving existing git syncing.

Written by the indexing model from the issue text.