Support detached HEAD in manifest repository

If the manifest repository is on a detached HEAD and we are parsing
an XML formatted manifest we should simply set the branch property
to None, rather than crash with an AttributeError.

Signed-off-by: Shawn O. Pearce <sop@google.com>

manifest_xml.py

@@ -183,7 +183,7 @@ class XmlManifest(object):
     if not self._loaded:
       m = self.manifestProject
       b = m.GetBranch(m.CurrentBranch).merge
-      if b.startswith(R_HEADS):
+      if b is not None and b.startswith(R_HEADS):
         b = b[len(R_HEADS):]
       self.branch = b