Evil Merge branch 'jc/status' (early part) into js/diff-ni

* 'jc/status' (early part):
  run_diff_{files,index}(): update calling convention.
  update-index: do not die too early in a read-only repository.
  git-status: do not be totally useless in a read-only repository.

This is to resolve semantic conflict (which is not textual) that
changes the calling convention of run_diff_files() early.

builtin-diff.c

@@ -117,6 +117,10 @@ static int builtin_diff_index(struct rev_info *revs,
 	    revs->max_count != -1 || revs->min_age != -1 ||
 	    revs->max_age != -1)
 		usage(builtin_diff_usage);
+	if (read_cache() < 0) {
+		perror("read_cache");
+		return -1;
+	}
 	return run_diff_index(revs, cached);
 }