do not merge stderr & stdout when checking ancestors

The ancestor check depends on finding some output. If we get an error
message, that shouldn't count.

Change-Id: If5c079249d7b32ac6a7418f853a7db06082593bc
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

openstack_releases/gitutils.py

@@ -88,7 +88,6 @@ def check_ancestry(workdir, repo, old_version, sha):
             ['git', 'log', '--oneline', '--ancestry-path',
              '%s..%s' % (old_version, sha)],
             cwd=os.path.join(workdir, repo),
-            stderr=subprocess.STDOUT,
         ).strip()
         return bool(ancestors)
     except subprocess.CalledProcessError as e: