gitserver: Fall back to GitRepoSyncer for missing repos

Created by: ryanslade

When checking if a repo is cloneable it may be a new repo that doesn't exist in our database yet. In this case we assume it's a git repo. This case can only currently occur on Cloud when adding "on demand" repos and on Cloud we only support git.

Should fix https://github.com/sourcegraph/sourcegraph/issues/17672