Add tests for plugins setting file content to None

t/plugins/removefiles_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'].startswith(b'bad'):
+            file_data['data'] = None