[RFC] Rename git-rm --cached to --index

Hello!

I just re-read the git-rm manpage and recognized git-rm knows about the --cached option. As in current git we do not talk about the cache, but about the index, it would imho make sense to rename --cache to --index in a user friendly way:

- add --index to git-rm
- add --index to manpage of git-rm
- still support --cached in git-rm
- add hint in manpage that --cached is old naming style

I think naming it --index would make life more consistent for new users of git.

Now I'm interested in your opinion ;-)

Sincerly

Nico

P.S.: As always, please CC me on reply