Support "repo init -b foo && repo sync" to switch baselines

We now correctly support re-initializing an existing client to point
to a different branch of the same manifest repository, effectively
allowing the client to switch the baseline it is operating on.

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

subcmds/init.py

@@ -126,7 +126,8 @@ default.xml will be used.
 
     m.Sync_NetworkHalf()
     m.Sync_LocalHalf()
-    m.StartBranch('default')
+    if is_new:
+      m.StartBranch('default')
 
   def _LinkManifest(self, name):
     if not name: