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>