vfs: add "writelines"

This patch allows "writelines" to take "mode" and "notindexed"
arguments, because subsequent patch for subrepo requires both.

mercurial/scmutil.py

@@ -229,6 +229,13 b' class abstractvfs(object):'
         finally:
             fp.close()
 
+    def writelines(self, path, data, mode='wb', notindexed=False):
+        fp = self(path, mode=mode, notindexed=notindexed)
+        try:
+            return fp.writelines(data)
+        finally:
+            fp.close()
+
     def append(self, path, data):
         fp = self(path, 'ab')
         try: