add option to show diff before asking to overwrite

tests/helpers.py

@@ -80,6 +80,7 @@ def load_config(confpath, profile):
     opts['safe'] = True
     opts['installdiff'] = True
     opts['link'] = False
+    opts['showdiff'] = True
     opts['debug'] = True
     opts['dopts'] = ''
     opts['variables'] = {}