gitserver: support cloning repos with a customFetchCmd

Created by: keegancsmith

The existing customer which uses customFetchCmd manually cloned the repository into gitserver, so the support for customFetchCmd previously didn't take into account the need to clone.

We only know how to fetch these repos, so instead of calling "git clone" we instead init an empty repository and then run fetch. I am unsure the wider implications of missing a remote URL in the git config / etc. However, the testing I did should unblock the customer relying on the need to clone.

Fixes https://github.com/sourcegraph/sourcegraph/issues/8875