pylint: fix indentation in manifest_xml

This fixes pylint warning:

************* Module manifest_xml
W:975, 0: Bad indentation. Found 8 spaces, expected 6 (bad-indentation)

Change-Id: I967212f9439430351836ebdc27e442d7b77476e2
Signed-off-by: Stefan Beller <sbeller@google.com>

manifest_xml.py

@@ -972,5 +972,5 @@ class GitcManifest(XmlManifest):
   def _output_manifest_project_extras(self, p, e):
     """Output GITC Specific Project attributes"""
     if p.old_revision:
-        e.setAttribute('old-revision', str(p.old_revision))
+      e.setAttribute('old-revision', str(p.old_revision))