[racket] Prevent dot directories from being planet-archived

A week ago, Jay McCarthy wrote:
> If you use the new package system with the standard deployment
> mode---git---then the only files that will be distributed are the ones
> that you explicitly add to the git repository. [...]

FWIW, it's very common to not distribute any ".git*" files that are
committed, since they're all meta.  Typical examples are .gitignore
and .gitattributes files.

In our main repository, for example, there is a .gitattributes file
that specifies these files as things to not distribute:

  .git* export-ignore
  /.mailmap export-ignore

It might not be too hard to DTRT and use the same specifications if
there is a git executable to use.  (If the code already depends on a
git executable and if you can easily plug such a command in, then I
can look up a command line to list files to be distributed that
ignores these things.)

-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                    http://barzilay.org/                   Maze is Life!