switch releasenotes job to branch-git-prep
switch releasenotes job to branch-git-prep

Use branch-git-prep instead of zuul-git-branch-prep to prepare the local
source repository, since it appears the latter does not work properly in
post jobs, yet.

Change-Id: I8cbcd49ff4bf7234e149985535f394bb686c58c3
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

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