Clone a single revision

Previously, when a buildpack was cloned, the entire buildpack repository was fetched.  This resulted in a slower clone than was otherwise necessary.  This change executes the clone with a `--depth 1` which only fetches a single revision and no history.  This results in noticeable speed gains.

| Buildpack | Full | Depth 1
| --------- | ---- | -------
| `cloudfoundry-buildpack-java` | `2.031s` | `1.451s`
| `heroku-buildpack-nodejs` | `6.840s` | `1.577s`
| `heroku-buildpack-ruby` | `5.075s` | `1.785s`

[#50080879]