Fix create_branch in tests

This method does not need to perform a 'git init'; the repo should
already exist.

Change-Id: I5e85a76d7c0eb85a15a59f7d4b648269c016feb2
diff --git a/tests/base.py b/tests/base.py
index c159865..df9fbc1 100755
--- a/tests/base.py
+++ b/tests/base.py
@@ -2419,7 +2419,7 @@
 
     def create_branch(self, project, branch):
         path = os.path.join(self.upstream_root, project)
-        repo = git.Repo.init(path)
+        repo = git.Repo(path)
         fn = os.path.join(path, 'README')
 
         branch_head = repo.create_head(branch)