Fix syntax mistake.

Author: mvousden

jobs/hook_container.yml

@@ -27,23 +27,14 @@
     executable: /bin/bash
   register: version
 
-- name: Obtain current branch of this repository. Does not work in detached-head state.
-  shell: git symbolic-ref HEAD | grep -o [^/]*$
-  args:
-    chdir: "{{ container_dir }}"
-    executable: /bin/bash
-  register: branch
-  when: ("{{ lookup('env','GIT_BRANCH') }}" == "")
-
 # Alternatively, we can obtain the branch in detached-head state using the
 # environment variable GIT_BRANCH if it is set. This helps out Jenkins.
-- name: Obtain current branch of this repository using GIT_BRANCH environment variable.
-  shell: echo $GIT_BRANCH
+- name: Obtain current branch of this repository.
+  shell: if [ -z "$GIT_BRANCH" ]; then git symbolic-ref HEAD | grep -o [^/]*$; else echo "$GIT_BRANCH"; fi
   args:
     chdir: "{{ container_dir }}"
     executable: /bin/bash
   register: branch
-  when: ("{{ lookup('env','GIT_BRANCH') }}" == "")
 
 # Exporting container filesystem as tarball.