Fix proposal operations

The proposals bot run on a tree that is in detached state. Current git
review fails to push from this any changes.

Create instead directly a branch and work on that.

Change-Id: I487337cb1def5f5995d776c831ea61b658652677

jenkins/scripts/common.sh

@@ -15,4 +15,8 @@ function setup_git {
     git config user.name "OpenStack Proposal Bot"
     git config user.email "openstack-infra@lists.openstack.org"
     git config gitreview.username "proposal-bot"
+
+    # Initial state of repository is detached, create a branch to work
+    # from. Otherwise git review will complain.
+    git checkout -B proposals
 }