Move get_head_sha into testing since it is only used by tests.

testing/util.py

@@ -2,6 +2,7 @@ import jsonschema
 import os
 import os.path
 import shutil
+from plumbum import local
 
 
 TESTING_DIR = os.path.abspath(os.path.dirname(__file__))
@@ -29,6 +30,11 @@ def copy_tree_to_path(src_dir, dest_dir):
             shutil.copy(srcname, destname)
 
 
+def get_head_sha(dir):
+    with local.cwd(dir):
+        return local['git']['rev-parse', 'HEAD']().strip()
+
+
 def is_valid_according_to_schema(obj, schema):
     try:
         jsonschema.validate(obj, schema)