Fix tgz archival on Windows.

Making tgz's on Windows has been broken by 06de65673ec2
due to not being careful enough about binary mode opens.

mercurial/archival.py

@@ -72,7 +72,7 b' class tarit:'
             if kind == 'gz':
                 mode = mode[0]
                 if not fileobj:
-                    fileobj = open(name, mode)
+                    fileobj = open(name, mode + 'b')
                 gzfileobj = self.GzipFileWithTime(name, mode + 'b',
                                                   zlib.Z_BEST_COMPRESSION,
                                                   fileobj, timestamp=mtime)