[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

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