Fix base query for empty diff

When the latest branch cut is from the very same hash as the previous
branch was cut from, then the rev-list gives an empty list. Originally
the last item from the list should have been taken. Since the list is
empty the splitlines()[-1] gives "IndexError: list index out of range".
This patch fixes this case with returning None early.

Change-Id: If5ea4a356b0c7620e2de1d2ab77951d8b76bb5e2

openstack_releases/gitutils.py

@@ -445,6 +445,10 @@ def get_branch_base(workdir, repo, branch):
         LOG.warning('failed to retrieve branch base: %s [%s]',
                     e, e.output.strip())
         return None
+    if parents == '':
+        LOG.warning('branch base query result was empty for %s repository',
+                    repo)
+        return None
     parent = parents.splitlines()[-1]
     # Now get the ^^! commit
     cmd = [