Issue a warning when a git external points to a branch

[billsacks]

When a git external points to a branch, manage_externals can say that you're in sync when really there have been changes on the remote that you haven't fetched yet (because we have decided that manage_externals should not do a fetch when you're asking for the status).

We should have manage_externals output a warning message when you ask for status and a git external points to a branch. For example, we could have some text at the bottom of the status output like:

************************************************************************
WARNING: The following git externals point to branches:
    XXXXXX

To ensure that you are getting an accurate status report, make sure you
have fetched any updates made on the remote, by going into the
appropriate external directories and issuing a 'git fetch'.
************************************************************************