sync: add sanity check for local checkouts missing network

If you run `repo sync -l foo` without first `repo sync -n foo`,
repo sets up an invalid gitdir tree that gets wedged and requires
manual recovery.  Add a sanity check to abort cleanly first.

Change-Id: Iad865ea860a3f1fd2f39ce683fe66bd4380745a5
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/244732
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

project.py

@@ -1499,6 +1499,13 @@ class Project(object):
     """Perform only the local IO portion of the sync process.
        Network access is not required.
     """
+    if not os.path.exists(self.gitdir):
+      syncbuf.fail(self,
+                   'Cannot checkout %s due to missing network sync; Run '
+                   '`repo sync -n %s` first.' %
+                   (self.name, self.name))
+      return
+
     self._InitWorkTree(force_sync=force_sync, submodules=submodules)
     all_refs = self.bare_ref.all
     self.CleanPublishedCache(all_refs)