Abstract manifest branch creation from init to the manifest object

This permits the XML style manifest to use 'default', while other
types can use their own creation strategy for the current branch.

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/manifest.py b/manifest.py
index 0762098..a2fc960 100644
--- a/manifest.py
+++ b/manifest.py
@@ -40,5 +40,8 @@
   def projects(self):
     return {}
 
+  def InitBranch(self):
+    pass
+
   def SetMRefs(self, project):
     pass