ph/parseopt-sh reloaded

Okay here is a fixed series wrt the security issue Junio raised. Instead of the old PARSEOPT_OPTS variable, I now only have OPTIONS_KEEPDASHDASH to be set to non empty if you want to add --keep-dashdash. The reason for that is that I dislike that every single git-rev-parse --parseopt user had to do a `PASREOPT_OPTS=` at the begining of each script, it's error prone, and ugly.

PARSEOPT_OPTS was an overkill as it wasn't really used for anything else than OPTIONS_KEEPDASHDASH, and if it has to be used for more, it'll be easy to extend the specification parser to take options on stdin rather than through parameters.

I also removed the PARSEOPT_OPTS from git-clone.sh as it was a spurious use, I don't intend users to override this variable, it's indeed an internal that changes git-rev-parse --parseopt behaviour in a incompatible way for the scripts that uses it, it should not be user-tweakable anyway.

The 10 patch series (and not 11, I forgot about 7 when I incrementally sent the previous one) is fetcheable from my repository:

  git://git.madism.org/git.git on branch ph/parseopt-sh

(ph/parseopt has the remaining patches that are problematic right now either because of the small change -h vs. -H or the patches that conflicts with git-fetch series right now, but in the spirit this is definitely a ph/parseopt series).

Cheers,