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_xml.py b/manifest_xml.py
index 66cdf3e..45896be 100644
--- a/manifest_xml.py
+++ b/manifest_xml.py
@@ -169,6 +169,12 @@
     self._Load()
     return self._default
 
+  def InitBranch(self):
+    m = self.manifestProject
+    if m.CurrentBranch is None:
+      return m.StartBranch('default')
+    return True
+
   def SetMRefs(self, project):
     if self.branch:
       project._InitAnyMRef(R_M + self.branch)