git-diff: Introduce --index and deprecate --cached.

git-diff: Introduce --index and deprecate --cached.

'git diff --cached' still works, but its use is discouraged
in the documentation. 'git diff --index' does the same thing
and is consistent with how 'git apply --index' works.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>