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>

manifest.py

@@ -40,5 +40,8 @@ class Manifest(object):
   def projects(self):
     return {}
 
+  def InitBranch(self):
+    pass
+
   def SetMRefs(self, project):
     pass

manifest_xml.py

@@ -169,6 +169,12 @@ class XmlManifest(Manifest):
     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)

subcmds/init.py

@@ -130,10 +130,9 @@ to update the working directory files.
     m.Sync_LocalHalf(syncbuf)
     syncbuf.Finish()
 
-    if is_new or m.CurrentBranch is None:
-      if not m.StartBranch('default'):
-        print >>sys.stderr, 'fatal: cannot create default in manifest'
-        sys.exit(1)
+    if not self.manifest.InitBranch():
+      print >>sys.stderr, 'fatal: cannot create branch in manifest'
+      sys.exit(1)
 
   def _LinkManifest(self, name):
     if not name: