[PATCH] Re-instate index file write optimization

This makes "git-update-index" avoid the new index file write if it didn't
make any changes to the index.

It still doesn't make things like "git status" be read-only operations in
general, but if the index file doesn't need refreshing, it now will at
least avoid making unnecessary changes.
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

update-index.c

@@ -391,9 +391,11 @@ int main(int argc, const char **argv)
 			update_one(buf.buf, prefix, prefix_length);
 		}
 	}
-	if (write_cache(newfd, active_cache, active_nr) ||
-	    commit_index_file(&cache_file))
-		die("Unable to write new cachefile");
+	if (active_cache_changed) {
+		if (write_cache(newfd, active_cache, active_nr) ||
+		    commit_index_file(&cache_file))
+			die("Unable to write new cachefile");
+	}
 
 	return has_errors ? 1 : 0;
 }