Check for existence of refs upon initial fetch

When we do an initial fetch and have not specified any branch etc,
the following fetch command will not error:
git fetch origin --tags +refs/heads/*:refs/remotes/origin/*

In this change we make sure something got fetched and if not we report
an error.

This fixes the bug that occurs when we init using a bad manifest url and
then are unable to init again (because a manifest project has been
inited with no manifest).

Change-Id: I6f8aaefc83a1837beb10b1ac90bea96dc8e61156

project.py

@@ -1748,6 +1748,7 @@ class Project(object):
         cmd.append('--no-tags')
       else:
         cmd.append('--tags')
+
       cmd.append(str((u'+refs/heads/*:') + remote.ToLocal('refs/heads/*')))
     elif tag_name is not None:
       cmd.append('tag')
@@ -1774,6 +1775,11 @@ class Project(object):
       time.sleep(random.randint(30, 45))
 
     if initial:
+      # Ensure that some refs exist.  Otherwise, we probably aren't looking
+      # at a real git repository and may have a bad url.
+      if not self.bare_ref.all:
+          ok = False
+
       if alt_dir:
         if old_packed != '':
           _lwrite(packed_refs, old_packed)