Run file_data_filter on deleted files

The `file_data_filter` method should be called when files are deleted.
In this case the `data` and `file_ctx` keys map to None. This is so
that a filter which modifies file names can apply the same name
transformations before files are deleted.

t/plugins/rename_file_test_plugin/__init__.py

@@ -0,0 +1,15 @@
+import subprocess
+import shlex
+import sys
+from mercurial import node
+
+def build_filter(args):
+    return Filter(args)
+
+class Filter:
+    def __init__(self, args):
+        self.filter_contents = shlex.split(args)
+
+    def file_data_filter(self,file_data):
+        if file_data['filename'] == b'b.txt':
+            file_data['filename'] = b'c.txt'