gitserver: add functionality to run git commands locally

Created by: sashaostrikov

This PR is another step towards making it possible to run tests without gitserver server instance running, which is needed for further refactoring (https://github.com/sourcegraph/sourcegraph/issues/33280)

This allows using GitCommand interface implementation (LocalGitCommand) for tests which don't need a running Gitserver instance.

Closes https://github.com/sourcegraph/sourcegraph/issues/34258

Test plan

New test case added