## #refex matching a ref and a refex

A refex is a word I made up to mean "a regex that matches a ref".  If you know
[regular expressions][regex] you're halfway there.

In addition:

  * if no refex is supplied, it defaults to `refs/.*`, for example in a rule
    like this:

        RW              =   alice

  * a refex not starting with `refs/` is assumed to start with `refs/heads/`.
    This means normal branches can be conveniently written like this:

        RW  master      =   alice
        # becomes 'refs/heads/master' internally

    while tags will need to be fully qualified

        RW  refs/tags/v[0-9]    =   bob

  * a refex is implicitly anchored at the start, but not at the end.  In
    regular expression lingo, a `^` is assumed at the start (but no `$` at the
    end is assumed).  So a refex of `master` will match all these:

        refs/heads/master
        refs/heads/master1
        refs/heads/master2
        refs/heads/master/full

    If you want to restrict the match to just the one specific ref, use

        RW  master$     =   alice