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

This reverts commit 4c81c213a4.

Although --cached and --index are confusing wording, the use of
word --cached for git-diff is consistent with git-apply.  It means
"work with index without looking at the working tree".

We should probably come up with better wording for --cached, if
somebody wants to deprecate it.  But making --index and --cached
synonyms for diff while leaving them mean different things for
apply is no good.

Documentation/git-diff.txt

@@ -22,10 +22,8 @@ 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
-  `--index` can be given to compare the index file and
+  `--cached` 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`.
@@ -49,7 +47,7 @@ Various ways to check your working tree::
 +
 ------------
 $ git diff            <1>
-$ git diff --index    <2>
+$ git diff --cached   <2>
 $ git diff HEAD       <3>
 ------------
 +

builtin-diff.c

@@ -137,7 +137,7 @@ static int builtin_diff_index(struct rev_info *revs,
 	int cached = 0;
 	while (1 < argc) {
 		const char *arg = argv[1];
-		if (!strcmp(arg, "--index") || !strcmp(arg, "--cached"))
+		if (!strcmp(arg, "--cached"))
 			cached = 1;
 		else
 			usage(builtin_diff_usage);