Make zuul-git-prep-upper-constraints safe for post
Make zuul-git-prep-upper-constraints safe for post

Extend zuul-git-prep-upper-constraints to infer a target branch from
$ZUUL_REFNAME when $ZUUL_BRANCH is not present. This works around
the absence of branch guidance for ref-updated triggers in post
pipeline jobs, where Zuul is not told a branch but Gerrit instead
uses the branch as the refname for the event.

Change-Id: I3fd06cc1f09847f6731f80deb00fd1515f0106e9

Tesora/tesora-project-config,dongwenjuan/project-config,noorul/os-project-config,openstack-infra/project-config,noorul/os-project-config,Tesora/tesora-project-config,dongwenjuan/project-config,openstack-infra/project-config