make rebase durable to failing again after rebase
make rebase durable to failing again after rebase

also fix pipefail masking the error

Signed-off-by: Chris Brown <eb0819782e2e967e3d4f4d71eab0c647ed2659cf@pivotal.io>

StartupMakers/ssh-resource,ivojinovic/pipeline-git-branch-resource,pivotaltracker/git-branches-resource,starkandwayne/git-multibranch-resource,vito/git-resource,concourse/git-resource,zachgersh/git-resource,barthy1/git-resource