Use fetch/checkout instead of pull.
Use fetch/checkout instead of pull.

Fixes bug 897871. Using pull can sometimes cause a merge.
Instead, we should fetch the change and then check that out.

Change-Id: I255d2fd8bc065105f9a5cf37f3520295c660c104

openstack-infra/git-review,meelapshah/git-review,ka2m/git-review,entropia1/git-review,meelapshah/git-review,jeeloo/git-review