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.

/reviewed-by @jerome, @kazuhiko
/reviewed-on !1

This fixes this error:
[2015-04-10 06:28:09,906] INFO         exec self.options['script']
[2015-04-10 06:28:09,906] INFO       File "<string>", line 6, in <module>
[2015-04-10 06:28:09,906] INFO     NameError: name 'env' is not defined

When installing busybox

This reverts commit ab3bdcfb.

This reverts commit 9bd1e7e7.
What about test nodes ??
Why treating unpushed commits differently ?

commited-but-not-pushed changes will be lost.

Test is still failing due to recent unrelated changes.

By default we do not want to risk destroying developer's work.