let's retry this based on git-rev-parse --parseopt

So as it seems git-rev-parse is the place to do that, here is a new proposal, with a few more commands migrated. Of course it doesn't means we should not rewrite some (if not all) of the shell scripts as builtins, but it will take some pain from the user, and provide nice option parsers from day 1 for new porcelains.

I believe that the git-am.sh migration is a perfect example of why this option parser helps.

Note that in the patch 1, the documentation _needs_ to be reviewed by a native speaker.