DiffTool: Fix for diff for multiline large text content

DiffTool depends on deepdiff library to calculate diff but this only
works for pure string cases. In all other cases, we manually create
unified diff from old and new value. This led to problem between
how to display diff for large multiline content and for single line
content.

Related issue on Bug Module: https://nexedi.erp5.net/bug_module/20180810-13FBEFD

product/ERP5/bootstrap/erp5_core/DocumentTemplateItem/portal_components/document.erp5.DiffTool.py

@@ -195,9 +195,16 @@ class PortalPatch:
         if (val.t2 == None) or isinstance(val.t2, deepdiff.helper.NotPresent):
           new_value = ''
 
+        # Deepdiff doesn't creates diff for anything other than string, thus for all other cases,
+        # we rely on difflib to maually create a diff.
         try:
-          # Create unified diff for single string cases
-          new_val['diff'] = ''.join(difflib.unified_diff([str(old_value)+'\n'], [str(new_value)+'\n'])).replace('\n', ' \n')
+          # If the old or new value is a multiline string, split the lines and then convert them
+          # to correct format of unified diff which can be displayed properly
+          if '\n' in str(old_value) or '\n' in str(old_value):
+            new_val['diff'] = ''.join(difflib.unified_diff(old_value.splitlines(1), new_value.splitlines(1))).replace('\n', ' \n')
+          else:
+            # Create unified diff for single string cases
+            new_val['diff'] = ''.join(difflib.unified_diff([str(old_value)+'\n'], [str(new_value)+'\n'])).replace('\n', ' \n')
         except ValueError:
           # Show empty diff in case of error in new or old value type
           new_val['diff'] = None