gitclone: We don't have to fetch, if revision is already present in local git repository
Because revision specifies worktree state exactly, and if we have it already, we don't need to fetch anything -- it is possible to checkout/reset to the revision right away.
The reason for this patch is that fetching is sometimes not very fast or even slow. For example today, gitlab.com had some outage and I've developed this patch while waiting for it to come back.