CBLT-84:  be sure to checkout {GITSPEC} branch before pulling

Change-Id: Ie9089d833ecb0a95d32d24a766ee39c1ac982832
Reviewed-on: http://review.couchbase.org/33292
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>