Utilize caching when fetching similar repos

**Describe the problem/challenge you have**
When it comes to using [overlapping sync options](https://github.com/vmware-tanzu/carvel-vendir/tree/develop/examples/overlapping-dir), it's usually that you would mention the same repo to pull from several times in order to substitute one of the folders with a custom one from the private repo, for instance:
```
directories:
- path: components
contents:
- path: terraform/tfstate-backend
git:
url: git@github.com:cloudpossy/terraform-aws-components.git
ref: all-new-components
newRootPath: modules/tfstate-backend
- path: terraform/account
git:
url: git@github.com:cloudpossy/terraform-aws-components.git
ref: all-new-components
newRootPath: modules/account
- path: terraform/account-map
git:
url: git@github.com:cloudpossy/terraform-aws-components.git
ref: all-new-components
newRootPath: modules/account-map
- path: terraform/nexus
git:
url: git@github.com:privateorg/terraform-aws-components.git
ref: atmos-novel
newRootPath: privateorg/modules/nexus
```

Here where it gets too long to wait when `vendir sync` is used because it basically tries to react on that planly by downloading `git@github.com:cloudpossy/terraform-aws-components.git` every time it's mentioned in the spec without attempting to re-use the cached sample.

I'd guess, that it's something of utmost importance to implement this simple functionality and make the tool much more efficient.

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