Allow repo init to restart if URL was initially invalid

This allows the user to run "repo init -u" again after an
initial attempt failed due to an invalid URL.

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/subcmds/init.py b/subcmds/init.py
index 04de48a..83ed2b6 100644
--- a/subcmds/init.py
+++ b/subcmds/init.py
@@ -130,7 +130,7 @@
       sys.exit(1)
 
     m.Sync_LocalHalf()
-    if is_new:
+    if is_new or m.CurrentBranch is None:
       m.StartBranch('default')
 
   def _LinkManifest(self, name):