add-submodules: Achieve OS X compatibility.

OS X doesn't use GNU grep, so it doesn't support the -P flag for Perl
compatible regular expressions (PCREs).  Found this trick online:

    http://stackoverflow.com/a/5694596

Previously it would fail on OS X with this message:

    usage: grep [-abcDEFGHhIiJLlmnOoPqRSsUVvwxZ] [-A num] [-B num] [-C[num]]
    [-e pattern] [-f file] [--binary-files=value] [--color=when]
    [--context[=num]] [--directories=action] [--label] [--line-buffered]
    [--null] [pattern] [file ...]

That's odd.  I swear I see a P right there between o and q.