CI: Add tests to avoid a repeat of #328

Extend tests to cover the file content filter example plugins in order
to avoid a repeat of #328.

t/file_data_filter.t

@@ -25,7 +25,9 @@ git_convert() {
 	cd "$2" &&
 	hg-fast-export.sh --repo "../$1" \
 			  -s --hgtags -n \
-			  --plugin ../../plugins/rename_file_test_plugin
+			  --plugin ../../plugins/rename_file_test_plugin \
+			  --plugin dos2unix \
+			  --plugin shell_filter_file_contents=../../plugins/id
 	)
 }
 

t/plugins/id

@@ -0,0 +1,2 @@
+#!/bin/bash
+cat