Git problem: "Failed to resolve 'HEAD' as a valid ref."

[David Moreau Simard]

The problem:
2014-02-19 15:01:38.954 | + git clone file:///opt/git/stackforge/puppet-swift .
2014-02-19 15:01:38.964 | Cloning into '.'...
2014-02-19 15:01:39.112 | warning: remote HEAD refers to nonexistent ref, unable to checkout.
2014-02-19 15:01:39.472 | + git reset --hard
2014-02-19 15:01:39.509 | fatal: Failed to resolve 'HEAD' as a valid ref.
2014-02-19 15:01:39.751 | Build step 'Execute shell' marked build as failure

Review: https://review.openstack.org/#/c/74082/
Failed merge: http://logs.openstack.org/82/74082/1/check/gate-puppet-swift-puppet-unit-3.3/f94e1ae/console.html

As a matter of comparison, the commit from which the above review was cherry-picked merged successfully:
Review: https://review.openstack.org/#/c/72693/
Successful merge: http://logs.openstack.org/50/73650/2/gate/gate-puppet-ceilometer-puppet-unit-3.3/7f6b25d/console.html

After talking to #openstack-infra, this seems to be something wrong with the git repos that are cached on the slave images.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/74780
Committed: https://git.openstack.org/cgit/openstack-infra/config/commit/?id=cbd64b61751bf22d2955ffb7f531a02947a97edb
Submitter: Jenkins
Branch: master

commit cbd64b61751bf22d2955ffb7f531a02947a97edb
Author: James E. Blair <email address hidden>
Date: Wed Feb 19 09:03:19 2014 -0800

    Add sync calls to all prepare scripts

    Strictly speaking, we only need sync calls at the end of the
    script that is directly invoked by nodepool. But just so we
    don't have to think about it too much, add them to all prepare
    scripts.

    Without it, clouds will happily snapshot a system without all
    the data written to disk!

    Closes-Bug: 1282136

    Change-Id: Id7dcf6667d6e48eb5ae95fd42b09f0651eeb38f9