Pre-load repositories already cloned to disk

You can use repositories that are already cloned to disk on the host machine to speed up Sourcegraph's cloning. This is useful for very large repositories on which cloning exceeds the resources available to the Docker container. This is not intended for individual users who want to set up a personal Sourcegraph instance just for searching code on their own local disk; we recommend either using a CLI tool such as ripgrep instead, or simply connecting Sourcegraph to your code host with a limited set of repositories.

The steps documented here are intended for Sourcegraph instances running on a single node. The general process also applies for clustered deployments of Sourcegraph to Kubernetes, but you need to perform these steps on the underlying node hosting the gitserver pod, or on the persistent volume used by the gitserver deployment.

If you're using the default --volume $HOME/.sourcegraph/data:/var/opt/sourcegraph argument to run the sourcegraph/server Docker image, and the repository you want to add is named github.com/my/repo, then follow these steps:

Ensure that the added repository is included in a code host configuration on Sourcegraph.
Stop Sourcegraph if it is running. This is to ensure it doesn't interfere with the clone.
On the host machine, ensure that a bare Git clone of the repository exists at $HOME/.sourcegraph/data/repos/github.com/my/repo/.git.

To create a new clone given its clone URL:
```
git clone --mirror YOUR-REPOSITORY-CLONE-URL $HOME/.sourcegraph/data/repos/github.com/my/repo/.git
```
Or, as an optimization, you can reuse an existing local clone to avoid needing to fetch all the repository data again:
```
git clone --mirror --reference PATH-TO-YOUR-EXISTING-LOCAL-CLONE --dissociate YOUR-REPOSITORY-CLONE-URL $HOME/.sourcegraph/data/repos/github.com/my/repo/.git
```

If this repository exists on a code host that Sourcegraph integrates with, then use that code host's configuration (as described in the code host documentation). After updating the code host configuration, if you used the correct repository path, Sourcegraph will detect and reuse the existing clone. (For example, if you're working with a repository on GitHub.com, ensure that the repository path name you used is of the form github.com/my/repo.)