apply: do not read from the filesystem under --index

We currently read the preimage to apply a patch from the index only
when the --cached option is given.  Do so also when the command is
running under the --index option.  With --index, the index entry and
the working tree file for a path that is involved in a patch must be
identical, so this should not affect the result, but by reading from
the index, we will get the protection to avoid reading an unintended
path beyond a symbolic link automatically.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/apply.c

@@ -3136,7 +3136,7 @@ static int load_patch_target(struct strbuf *buf,
 			     const char *name,
 			     unsigned expected_mode)
 {
-	if (cached) {
+	if (cached || check_index) {
 		if (read_file_or_gitlink(ce, buf))
 			return error(_("read of %s failed"), name);
 	} else if (name) {