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>

Documentation/git-diff.txt

@@ -22,8 +22,10 @@ the number of trees given to the command.
 
 * When one <tree-ish> is given, the working tree and the named
   tree are compared, using `git-diff-index`.  The option
-  `--cached` can be given to compare the index file and
+  `--index` can be given to compare the index file and
   the named tree.
+  `--cached` is a deprecated alias for `--index`. It's use is
+  discouraged.
 
 * When two <tree-ish>s are given, these two trees are compared
   using `git-diff-tree`.
@@ -47,7 +49,7 @@ Various ways to check your working tree::
 +
 ------------
 $ git diff            <1>
-$ git diff --cached   <2>
+$ git diff --index    <2>
 $ git diff HEAD       <3>
 ------------
 +